Practice Strings

Topics
  1. Reverse and it’s Variants
  2. Palindrome and it’s Variants
  3. Vowel Count and it’s Variants
  4. Anagrams and it’s Variants
  5. Count of occurrences and it’s Variants

Practice Exercises

1. Reverse a String

Problem: Write a method to reverse a given string.

Solution 
   public String reverse(String str) {
       String reversed = "";
       for (int i = str.length() - 1; i >= 0; i--) {
           reversed = reversed + str.charAt(i);
       }
       return reversed;
   }

Variants

  1. Reverse a Sentence (Keep Words Intact)
    Problem: Write a method to reverse a sentence, but keep each word in its original order. For example, "Hello World" should become "World Hello".
  2. Reverse Words Individually in a Sentence
    Problem: Reverse each word individually in a sentence. For example, "Hello World" should become "olleH dlroW".
  3. Reverse Only Vowels in a String
    Problem: Reverse only the vowels in a string while keeping consonants in their original position. For example, "hello" should become "holle".

2. Check if a String is a Palindrome

Problem: Write a method to check if a given string is a palindrome (a word that reads the same forward and backward).

Solution 
   public boolean isPalindrome(String str) {
       int start = 0;
       int end = str.length() - 1;
       while (start < end) {
           if (str.charAt(start) != str.charAt(end)) {
               return false;
           }
           start++;
           end--;
       }
       return true;
   }

Variants

  1. Case-Insensitive Palindrome Check
    Problem: Modify the original method to perform a case-insensitive palindrome check. "Madam" should be considered a palindrome.
  2. Palindrome Check Ignoring Non-Alphanumeric Characters
    Problem: Check if a string is a palindrome, ignoring spaces, punctuation, and non-alphanumeric characters. "A man, a plan, a canal: Panama" should return true.
  3. Check Palindrome Recursively
    Problem: Write a recursive method to check if a string is a palindrome.

3. Count the Number of Vowels in a String

Problem: Write a method that counts and returns the number of vowels in a string.

Solution 
    public int countVowels(String str) {
        int count = 0;
        for (char c : str.toLowerCase().toCharArray()) {
            if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') {
                count++;
            }
        }
        return count;
    }

Variants

  1. Count Vowels and Consonants Separately
    Problem: Write a method that counts and returns both the number of vowels and consonants in a string.
  2. Count Uppercase and Lowercase Vowels Separately
    Problem: Write a method that counts and returns both the number of uppercase and lowercase vowels in a string.
  3. Count Vowels Using Streams
    Problem: Write a method that counts the number of vowels using Java Streams.

4. Check if Two Strings are Anagrams

Problem: Write a method to check if two strings are anagrams (contain the same characters in a different order).

Solution 
   public 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);
   }

Variants

  1. Case-Insensitive Anagram Check
    Problem: Modify the original method to check for anagrams in a case-insensitive manner. "Listen" and "Silent" should be considered anagrams.
  2. Anagram Check Ignoring Non-Alphanumeric Characters
    Problem: Modify the method to check if two strings are anagrams, ignoring spaces, punctuation, and non-alphanumeric characters. "Tom Marvolo Riddle" and "I am Lord Voldemort" should return true.
  3. Anagram Check Using Frequency Array
    Problem: Write an optimized method to check if two strings are anagrams by counting character frequencies in a single pass.

5. Count the Occurrences of a Substring

Problem: Write a method that counts how many times a substring appears in a string.

Solution 
   public int countSubstring(String str, String sub) {
      int count = 0;
      int index = 0;
      while ((index = str.indexOf(sub, index)) != -1) {
         count++;
         index += sub.length();
      }
      return count;
   }

Variants

  1. Count Overlapping Substring Occurrences
    Problem: Write a method to count how many times a substring appears in a string, including overlapping occurrences. For example, "aa" in "aaaa" appears 3 times.
  2. Count Substring Occurrences with Case Insensitivity
    Problem: Modify the method to count how many times a substring appears in a string, ignoring case. For example, "Java" in "I love java and JAVA" should return 2.
  3. Count Substring Occurrences with Regex
    Problem: Write a method that uses regular expressions to count the number of occurrences of a substring in a string.

These variations will challenge you to think critically about string manipulation and develop a deep understanding of the different ways strings can be processed in Java.

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