Important Questions

1.            Reverse of string?

public class ReverseString {

    public static void main(String[] args) {

        String str = "Hello";

                             StringBuilder sb = new StringBuilder(str);

                                           // Sting builder is  function that can access the Sting

        System.out.println("Reversed String: " + sb.reverse().toString());

   //Reverse is a methods that can implement the  reverse of String

//toSting() it convert into the String

    }

}

2.   2.  Check if String is palindrome?

 

public class PalindromeCheck {

    public static void main(String[] args) {

        String str = "madam";

        System.out.println(isPalindrome(str) ? "Palindrome" : "Not a Palindrome");

    }

 

    public static boolean isPalindrome(String str) {

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

        return str.equals(reversed);

//show that same String from Right and Left Direction.

    }

}

3.Remove Duplicate from a String?

 

import java.util.LinkedHashSet;

 

public class RemoveDuplicates {

    public static void main(String[] args) {

        String str = "programming";

        System.out.println("Without Duplicates: " + removeDuplicates(str));

    }

 

    public static String removeDuplicates(String str) {

        LinkedHashSet<Character> set = new LinkedHashSet<>();

        for (char ch : str.toCharArray()) {

            set.add(ch);  // Adds unique characters only

        }

        StringBuilder sb = new StringBuilder();

        for (char ch : set) {

            sb.append(ch);

        }

        return sb.toString();

    }

}

  4. Find the first Non-Repeation Character

 

import java.util.LinkedHashMap;

import java.util.Map;

 

public class FirstNonRepeating {

    public static void main(String[] args) {

        String str = "swiss";

        System.out.println("First Non-Repeating Character: " + firstNonRepeating(str));

    }

 

    public static char firstNonRepeating(String str) {

        Map<Character, Integer> charCount = new LinkedHashMap<>();

 

        // Count character occurrences

        for (char ch : str.toCharArray()) {

            charCount.put(ch, charCount.getOrDefault(ch, 0) + 1);

        }

 

        // Find the first character with count 1

        for (Map.Entry<Character, Integer> entry : charCount.entrySet()) {

            if (entry.getValue() == 1) {

                return entry.getKey();

            }

        }

 

        return '\0'; // Return null character if no unique character found

    }

}

5.Count the occurrence of Each Character?

import java.util.HashMap;

 

public class CharacterCount {

    public static void main(String[] args) {

        String str = "hello world";

        countCharacters(str);

    }

 

    public static void countCharacters(String str) {

        HashMap<Character, Integer> charCount = new HashMap<>();

 

        // Count character occurrences

        for (char ch : str.toCharArray()) {

            if (ch != ' ') { // Ignoring spaces

                charCount.put(ch, charCount.getOrDefault(ch, 0) + 1);

            }

        }

 

        // Print character counts

        for (char ch : charCount.keySet()) {

            System.out.println(ch + ": " + charCount.get(ch));

        }

    }

}

6.Reverse of a word?

public class ReverseTwoWords {

    public static void main(String[] args) {

        String str = "Hello World";

        System.out.println("Reversed Words: " + reverseEachWord(str));

    }

 

    public static String reverseEachWord(String str) {

        String[] words = str.split(" ");

        return new StringBuilder(words[0]).reverse() + " " + new StringBuilder(words[1]).reverse();

    }

}

7.Check if Two Stings are Anagrams?

 

import java.util.Arrays;

 

public class AnagramCheck {

    public static void main(String[] args) {

        String str1 = "listen";

        String str2 = "silent";

       

        System.out.println("Are Anagrams? " + areAnagrams(str1, str2));

    }

 

    public static boolean areAnagrams(String str1, String str2) {

        if (str1.length() != str2.length()) return false;

 

        char[] arr1 = str1.toCharArray();

        char[] arr2 = str2.toCharArray();

       

        Arrays.sort(arr1);

        Arrays.sort(arr2);

       

        return Arrays.equals(arr1, arr2);

    }

}

8.Find the Longest Substring Without      Repeating Characters?

 

import java.util.HashSet;

 

public class LongestSubstring {

    public static void main(String[] args) {

        String str = "abcabcbb";

        System.out.println("Length of Longest Substring: " + longestUniqueSubstring(str));

    }

 

    public static int longestUniqueSubstring(String str) {

        int left = 0, right = 0, maxLength = 0;

        HashSet<Character> uniqueChars = new HashSet<>();

 

        while (right < str.length()) {

            if (!uniqueChars.contains(str.charAt(right))) {

                uniqueChars.add(str.charAt(right));

                maxLength = Math.max(maxLength, right - left + 1);

                right++;

            } else {

                uniqueChars.remove(str.charAt(left));

                left++;

            }

        }

        return maxLength;

    }

}

 

9.Convert a String to an integer?

 

public class StringToInteger {

    public static void main(String[] args) {

        String str = "12345";

        int num = Integer.parseInt(str);

        System.out.println("Converted Integer: " + num);

    }

}

 

10.Compress a String (Run- Length Encoding)?

public class RunLengthEncoding {

    public static void main(String[] args) {

        String str = "aaabbcddd";

        System.out.println("Compressed String: " + compressString(str));

    }

 

    public static String compressString(String str) {

        if (str == null || str.isEmpty()) return "";

 

        StringBuilder compressed = new StringBuilder();

        int count = 1;

 

        for (int i = 1; i < str.length(); i++) {

            if (str.charAt(i) == str.charAt(i - 1)) {

                count++;

            } else {

                compressed.append(str.charAt(i - 1)).append(count);

                count = 1;

            }

        }

        compressed.append(str.charAt(str.length() - 1)).append(count);

 

        return compressed.toString();

    }

}