HSE HackerRank Coding Patterns

HSE (HackerRank Software Engineering Assessment) is one of the toughest ILP exams — conducted on the HackerRank platform. REST APIs and Java coding are known to be important. Exact format may vary by batch. This page gives you coding templates and patterns to prepare.

1. How HackerRank Works

HackerRank runs your Java code against multiple hidden test cases. Your job is simple:

2. HackerRank Java Template (MEMORIZE THIS)

This is the skeleton you type first for every HSE problem. Memorize it cold.

import java.util.*;
import java.io.*;

public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        // Step 1: Read input

        // Step 2: Process / solve

        // Step 3: Print output

        sc.close();
    }
}

3. Input Reading Patterns (CRITICAL)

This section alone will save you from 80% of HSE failures. Most students know the logic but can't read input correctly.

Read a Single Integer

int n = sc.nextInt();

Read a Single Long

long n = sc.nextLong();

Read a Single Double

double d = sc.nextDouble();

Read a Single Word (no spaces)

String word = sc.next();  // reads until whitespace

Read an Entire Line (with spaces)

String line = sc.nextLine();  // reads entire line including spaces

// BROKEN — nextLine() reads the leftover \n
int n = sc.nextInt();
String name = sc.nextLine();  // name = "" (empty!)

// FIXED — consume the leftover \n first
int n = sc.nextInt();
sc.nextLine();  // eat the leftover newline
String name = sc.nextLine();  // now reads correctly

Read N Integers into an Array

int n = sc.nextInt();
int[] arr = new int[n];
for (int i = 0; i < n; i++) {
    arr[i] = sc.nextInt();
}

Read a Line of Space-Separated Integers

String line = sc.nextLine();
String[] parts = line.split(" ");
int[] arr = new int[parts.length];
for (int i = 0; i < parts.length; i++) {
    arr[i] = Integer.parseInt(parts[i]);
}

Read Multiple Lines of Input

int n = sc.nextInt();
sc.nextLine();  // consume leftover newline!
String[] lines = new String[n];
for (int i = 0; i < n; i++) {
    lines[i] = sc.nextLine();
}

Read Until End of File (EOF)

while (sc.hasNext()) {
    String line = sc.nextLine();
    // process each line
}

// Or for integers:
while (sc.hasNextInt()) {
    int num = sc.nextInt();
    // process each number
}

// Input:
// 3
// Alice 85
// Bob 92
// Charlie 78

int n = sc.nextInt();
sc.nextLine();  // consume newline after the integer!

for (int i = 0; i < n; i++) {
    String line = sc.nextLine();
    String[] parts = line.split(" ");
    String name = parts[0];
    int score = Integer.parseInt(parts[1]);
}

4. Type Conversion Patterns (Always Needed)

Almost every HSE problem requires converting between types. Memorize these — you'll use them every single time.

double d = 3.99;
int i = (int) d;          // i = 3 (TRUNCATED, not rounded!)
int j = (int) Math.round(d);  // j = 4 (rounded correctly)

int a = 7, b = 2;
System.out.println(a / b);         // prints 3 (NOT 3.5!)
System.out.println((double) a / b); // prints 3.5 (correct)
System.out.println(a * 1.0 / b);    // prints 3.5 (also correct)

5. Floating Point Precision (Frequently Tested)

Many HSE problems ask you to print results to a specific number of decimal places. Get this wrong and every test case fails.

Method 1: String.format() — RECOMMENDED

double value = 3.14159265;
String result = String.format("%.2f", value);  // "3.14"
System.out.println(result);

Method 2: System.out.printf()

double value = 3.14159265;
System.out.printf("%.2f%n", value);  // prints 3.14
System.out.printf("%.4f%n", value);  // prints 3.1416 (rounds!)

Method 3: Math.round() for manual rounding

double value = 3.14159;
double rounded = Math.round(value * 100.0) / 100.0;  // 3.14

6. String Manipulation Patterns

String problems are the most common in HSE. Know these methods cold.

Reverse a String

String reversed = new StringBuilder(str).reverse().toString();

Check Palindrome

String clean = str.toLowerCase().replaceAll("[^a-z0-9]", "");
String reversed = new StringBuilder(clean).reverse().toString();
boolean isPalindrome = clean.equals(reversed);

Count Character Occurrences

int count = 0;
for (char c : str.toCharArray()) {
    if (c == target) count++;
}

Count Vowels

int vowels = 0;
for (char c : str.toLowerCase().toCharArray()) {
    if ("aeiou".indexOf(c) != -1) vowels++;
}

String a = new String("hello");
String b = new String("hello");
System.out.println(a == b);       // false (compares references!)
System.out.println(a.equals(b));  // true (compares content)

7. Array Patterns

Find Maximum

int max = arr[0];
for (int i = 1; i < arr.length; i++) {
    if (arr[i] > max) max = arr[i];
}

Find Minimum

int min = arr[0];
for (int i = 1; i < arr.length; i++) {
    if (arr[i] < min) min = arr[i];
}

Sum of Array

int sum = 0;
for (int num : arr) {
    sum += num;
}

Average of Array

double avg = 0;
for (int num : arr) avg += num;
avg /= arr.length;  // avg is double, so no integer division issue

Reverse an Array

for (int i = 0, j = arr.length - 1; i < j; i++, j--) {
    int temp = arr[i];
    arr[i] = arr[j];
    arr[j] = temp;
}

Sort an Array

Arrays.sort(arr);  // ascending order

// For descending, sort then reverse — or use Integer[] with Comparator:
Integer[] boxed = Arrays.stream(arr).boxed().toArray(Integer[]::new);
Arrays.sort(boxed, Collections.reverseOrder());

Find Second Largest (Single Pass)

int first = Integer.MIN_VALUE, second = Integer.MIN_VALUE;
for (int num : arr) {
    if (num > first) {
        second = first;
        first = num;
    } else if (num > second && num != first) {
        second = num;
    }
}

Remove Duplicates (Using HashSet)

Set<Integer> unique = new LinkedHashSet<>();
for (int num : arr) unique.add(num);
// unique now contains only distinct values in insertion order

8. Common Problem Types in HSE

Pattern 1: String Processing

Input is typically a string. You transform it, count characters, validate format, or extract parts.

// Template: Read string, process, print result
Scanner sc = new Scanner(System.in);
String input = sc.nextLine();

// Examples of what they ask:
// - Count vowels/consonants
// - Reverse words in a sentence
// - Check if palindrome
// - Check if two strings are anagrams
// - Find longest word
// - Remove duplicate characters

Pattern 2: Array/Number Processing

Input is N followed by N numbers. You compute something from the array.

// Template: Read array, process, print result
Scanner sc = new Scanner(System.in);
int n = sc.nextInt();
int[] arr = new int[n];
for (int i = 0; i < n; i++) arr[i] = sc.nextInt();

// Examples of what they ask:
// - Find second largest
// - Find pairs that sum to target
// - Sort in custom order
// - Find duplicates
// - Compute running average

Pattern 3: Math Problems

// Factorial
long factorial(int n) {
    long result = 1;
    for (int i = 2; i <= n; i++) result *= i;
    return result;
}

// Fibonacci (first N terms)
int a = 0, b = 1;
for (int i = 0; i < n; i++) {
    System.out.print(a + " ");
    int temp = a + b;
    a = b;
    b = temp;
}

// Prime check
boolean isPrime(int n) {
    if (n < 2) return false;
    for (int i = 2; i <= Math.sqrt(n); i++) {
        if (n % i == 0) return false;
    }
    return true;
}

// GCD (Euclidean algorithm)
int gcd(int a, int b) {
    while (b != 0) {
        int temp = b;
        b = a % b;
        a = temp;
    }
    return a;
}

// LCM
int lcm(int a, int b) {
    return a / gcd(a, b) * b;  // divide first to avoid overflow
}

// Sum of digits
int sumOfDigits(int n) {
    int sum = 0;
    n = Math.abs(n);  // handle negative
    while (n > 0) {
        sum += n % 10;
        n /= 10;
    }
    return sum;
}

// Reverse a number
int reverseNumber(int n) {
    int rev = 0;
    while (n != 0) {
        rev = rev * 10 + n % 10;
        n /= 10;
    }
    return rev;
}

9. JSON / API Processing (VERY IMPORTANT for HSE)

This pattern catches everyone off-guard. HSE can give you a URL and ask you to fetch data from an API, parse the JSON response, and extract specific fields. Here's the complete template.

Complete HTTP GET + JSON Parse Template

import java.util.*;
import java.io.*;
import java.net.*;

public class Solution {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        String city = sc.nextLine().trim();

        // Step 1: Build the URL
        String urlStr = "https://api.example.com/data?city=" +
            URLEncoder.encode(city, "UTF-8");

        // Step 2: Make HTTP GET request
        URL url = new URL(urlStr);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");

        // Step 3: Read the response
        BufferedReader reader = new BufferedReader(
            new InputStreamReader(conn.getInputStream())
        );
        StringBuilder response = new StringBuilder();
        String line;
        while ((line = reader.readLine()) != null) {
            response.append(line);
        }
        reader.close();

        // Step 4: Parse JSON manually (no external library)
        String json = response.toString();

        // Extract a simple field value from JSON
        // For {"name":"Alice","age":25}
        // Find "name":"
        int start = json.indexOf("\"name\":\"") + 8;
        int end = json.indexOf("\"", start);
        String name = json.substring(start, end);

        System.out.println(name);
        sc.close();
    }
}

If org.json IS Available (Easier)

import org.json.*;

// Parse a JSON object
JSONObject json = new JSONObject(response.toString());
String name = json.getString("name");
int age = json.getInt("age");
double score = json.getDouble("score");

// Parse a JSON array
JSONArray items = json.getJSONArray("items");
for (int i = 0; i < items.length(); i++) {
    JSONObject item = items.getJSONObject(i);
    System.out.println(item.getString("title"));
}

// Handle nested objects
JSONObject address = json.getJSONObject("address");
String street = address.getString("street");

// Handle pagination (common pattern)
int page = 1;
int totalPages = 1;
while (page <= totalPages) {
    URL pageUrl = new URL(baseUrl + "&page=" + page);
    // ... fetch and parse ...
    totalPages = json.getInt("total_pages");
    page++;
}

10. Constructor Patterns (from FA, Tested in HSE)

Constructor questions in HSE usually appear as output prediction — they give you code with inheritance and constructors, and you predict what gets printed.

Key Rules

• If you don't write any constructor, Java adds a default no-arg constructor automatically
• If you write ANY constructor (even parameterized), Java does NOT add the default one
• this() calls another constructor in the same class — must be the FIRST statement
• super() calls the parent constructor — must be the FIRST statement
• If you don't write super(), Java adds super() (no-arg) automatically
• Constructor calls go UP the chain (child → parent → grandparent) but EXECUTE top-down

Output Prediction Example

class Animal {
    Animal() { System.out.println("Animal"); }
}
class Dog extends Animal {
    Dog() { System.out.println("Dog"); }
}
class Puppy extends Dog {
    Puppy() { System.out.println("Puppy"); }
}

// new Puppy() prints:
// Animal    (grandparent first)
// Dog       (parent second)
// Puppy     (child last)

class Parent {
    Parent(int x) { System.out.println("Parent: " + x); }
}
class Child extends Parent {
    Child() { System.out.println("Child"); }  // COMPILE ERROR!
}

11. Exception Handling Patterns

Basic try-catch

try {
    int result = Integer.parseInt("abc");  // throws NumberFormatException
} catch (NumberFormatException e) {
    System.out.println("Invalid number: " + e.getMessage());
}

Multiple catch blocks (ORDER MATTERS)

try {
    int[] arr = {1, 2, 3};
    System.out.println(arr[5]);
} catch (ArrayIndexOutOfBoundsException e) {
    System.out.println("Index out of range");
} catch (Exception e) {
    System.out.println("Some other error");
}
// Specific exceptions MUST come BEFORE general ones
// Putting Exception first = compile error (unreachable catch)

finally block

try {
    int x = 10 / 0;
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero");
} finally {
    System.out.println("Always executes");  // ALWAYS runs
}

12. Collections Shortcuts

ArrayList

ArrayList<Integer> list = new ArrayList<>();
list.add(10);          // add element
list.add(0, 5);       // insert at index 0
list.get(0);           // get element at index → 5
list.set(0, 99);       // replace element at index
list.remove(0);        // remove by index
list.size();            // number of elements
list.contains(10);     // true/false
Collections.sort(list);       // sort ascending
Collections.reverse(list);    // reverse
Collections.max(list);        // maximum
Collections.min(list);        // minimum
Collections.frequency(list, 10); // count of 10

HashMap

HashMap<String, Integer> map = new HashMap<>();
map.put("Alice", 90);       // add key-value
map.get("Alice");            // get value → 90
map.getOrDefault("Bob", 0); // get or return default
map.containsKey("Alice");   // true
map.containsValue(90);      // true
map.remove("Alice");         // remove entry
map.size();                  // number of entries

// Iterate over entries
for (Map.Entry<String, Integer> entry : map.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}

// Character frequency — MOST COMMON USE CASE
HashMap<Character, Integer> freq = new HashMap<>();
for (char c : str.toCharArray()) {
    freq.put(c, freq.getOrDefault(c, 0) + 1);
}

HashSet

HashSet<Integer> set = new HashSet<>();
set.add(10);         // add (ignores duplicates)
set.contains(10);    // true
set.remove(10);      // remove
set.size();           // count of unique elements

13. Output Formatting

printf Format Specifiers

System.out.printf("Name: %s, Age: %d, GPA: %.2f%n", name, age, gpa);

// %s  → string
// %d  → integer (int, long)
// %f  → floating point (double, float)
// %.2f → float with 2 decimal places
// %n  → platform-independent newline
// %10d → right-align integer in 10-char width
// %-10s → left-align string in 10-char width

Building Output with StringBuilder

// For multiple outputs, StringBuilder is faster than string concatenation
StringBuilder sb = new StringBuilder();
for (int i = 0; i < n; i++) {
    sb.append(arr[i]);
    if (i < n - 1) sb.append(" ");  // no trailing space
}
System.out.println(sb.toString());

14. Common Traps and Gotchas

15. 10 Practice Problems with Complete Solutions

These are the exact patterns that appear in TCS ILP exams. Type out each solution by hand at least once — don't just read them.

Problem 1: Average of N Numbers (2 Decimal Places)

5
10 20 30 40 50

30.00

import java.util.*;

public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int sum = 0;
        for (int i = 0; i < n; i++) {
            sum += sc.nextInt();
        }
        double avg = (double) sum / n;
        System.out.printf("%.2f%n", avg);
        sc.close();
    }
}

Problem 2: Palindrome Check (Case Insensitive)

RaceCar

Yes

import java.util.*;

public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.nextLine().toLowerCase();
        String rev = new StringBuilder(str).reverse().toString();
        System.out.println(str.equals(rev) ? "Yes" : "No");
        sc.close();
    }
}

Problem 3: Character Frequency

hello world

h:1
e:1
l:3
o:2
w:1
r:1
d:1

import java.util.*;

public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.nextLine().replaceAll("\\s+", "");

        // LinkedHashMap preserves insertion order
        LinkedHashMap<Character, Integer> freq = new LinkedHashMap<>();
        for (char c : str.toCharArray()) {
            freq.put(c, freq.getOrDefault(c, 0) + 1);
        }

        for (Map.Entry<Character, Integer> entry : freq.entrySet()) {
            System.out.println(entry.getKey() + ":" + entry.getValue());
        }
        sc.close();
    }
}

Problem 4: Second Largest in Array

5
12 35 1 10 34

34

import java.util.*;

public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int first = Integer.MIN_VALUE;
        int second = Integer.MIN_VALUE;

        for (int i = 0; i < n; i++) {
            int num = sc.nextInt();
            if (num > first) {
                second = first;
                first = num;
            } else if (num > second && num != first) {
                second = num;
            }
        }
        System.out.println(second);
        sc.close();
    }
}

Problem 5: Reverse Words in a Sentence

Hello World Java

Java World Hello

import java.util.*;

public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String[] words = sc.nextLine().split(" ");

        StringBuilder sb = new StringBuilder();
        for (int i = words.length - 1; i >= 0; i--) {
            sb.append(words[i]);
            if (i > 0) sb.append(" ");
        }
        System.out.println(sb.toString());
        sc.close();
    }
}

Problem 6: JSON/API Data Extraction (Template)

Dallas

12 degree

import java.util.*;
import java.io.*;
import java.net.*;

public class Solution {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        String city = sc.nextLine().trim();

        // Build URL
        String urlStr = "https://jsonmock.hackerrank.com/api/weather?name="
            + URLEncoder.encode(city, "UTF-8");

        // Make HTTP request
        URL url = new URL(urlStr);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");

        BufferedReader reader = new BufferedReader(
            new InputStreamReader(conn.getInputStream())
        );
        StringBuilder response = new StringBuilder();
        String line;
        while ((line = reader.readLine()) != null) {
            response.append(line);
        }
        reader.close();

        // Manual JSON parsing — extract "weather" value
        String json = response.toString();
        String key = "\"weather\":\"";
        int start = json.indexOf(key) + key.length();
        int end = json.indexOf("\"", start);
        String weather = json.substring(start, end);

        System.out.println(weather);
        sc.close();
    }
}

Problem 7: Sum of Digits

12345

15

import java.util.*;

public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = Math.abs(sc.nextInt());
        int sum = 0;
        while (n > 0) {
            sum += n % 10;  // get last digit
            n /= 10;       // remove last digit
        }
        System.out.println(sum);
        sc.close();
    }
}

Problem 8: Anagram Check

Listen
Silent

Anagrams

import java.util.*;

public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        char[] a = sc.nextLine().toLowerCase().toCharArray();
        char[] b = sc.nextLine().toLowerCase().toCharArray();

        Arrays.sort(a);
        Arrays.sort(b);

        if (Arrays.equals(a, b)) {
            System.out.println("Anagrams");
        } else {
            System.out.println("Not Anagrams");
        }
        sc.close();
    }
}

Problem 9: Find Duplicates in Array

7
4 2 7 2 4 9 7

2 4 7

import java.util.*;

public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        Set<Integer> seen = new HashSet<>();
        Set<Integer> duplicates = new LinkedHashSet<>();

        for (int i = 0; i < n; i++) {
            int num = sc.nextInt();
            if (!seen.add(num)) {  // add returns false if already exists
                duplicates.add(num);
            }
        }

        if (duplicates.isEmpty()) {
            System.out.println("No duplicates");
        } else {
            StringBuilder sb = new StringBuilder();
            for (int d : duplicates) {
                if (sb.length() > 0) sb.append(" ");
                sb.append(d);
            }
            System.out.println(sb.toString());
        }
        sc.close();
    }
}

Problem 10: Temperature Conversion

36.5

97.70

import java.util.*;

public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double celsius = sc.nextDouble();
        double fahrenheit = celsius * 9.0 / 5.0 + 32;
        System.out.printf("%.2f%n", fahrenheit);
        sc.close();
    }
}

Quick Reference: Copy-Paste Starters

import java.util.*;
public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] arr = new int[n];
        for (int i = 0; i < n; i++) arr[i] = sc.nextInt();
        // solve here
        sc.close();
    }
}

import java.util.*;
public class Solution {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.nextLine();
        // solve here
        sc.close();
    }
}

import java.util.*;
import java.io.*;
import java.net.*;
public class Solution {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        String input = sc.nextLine().trim();
        URL url = new URL("https://api.example.com/data?q=" + input);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        BufferedReader reader = new BufferedReader(
            new InputStreamReader(conn.getInputStream()));
        StringBuilder resp = new StringBuilder();
        String line;
        while ((line = reader.readLine()) != null) resp.append(line);
        reader.close();
        String json = resp.toString();
        // parse json and print result
        sc.close();
    }
}