Reverse a String in Java - The Coding Shala

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Reverse a String in Java

In the following post, we will see how to reverse a string in Java. Write a function that reverses a string. The input string is given as an array of characters char[].

Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory. You may assume all the characters consist of printable ASCII characters.

Example 1:

Input: ["h","e","l","l","o"]

Output: ["o","l","l","e","h"]

Example 2:

Input: ["H","a","n","n","a","h"]

Output: ["h","a","n","n","a","H"]

Reverse a String in Java Program

Method 1:

We can reverse a string using two pointers.

Java code:

class Solution {
    public void reverseString(char[] s) {
        int i = 0;
        int j = s.length-1;
        while(i<j){
            char tmp = s[i];
            s[i] = s[j];
            s[j] = tmp;
            i++;
            j--;
        }
    }
}

Method 2:

We can reverse a string using recursion.

Java Code:

class Solution {
    public void reverseString(char[] s) {
        String str = new String(s);
        str = reverse(str);
        for(int i =0; i<str.length(); i++) s[i] = str.charAt(i);
    }
    
    public String reverse(String str){
        int length = str.length();
        if (length <= 1) return str;
        String leftStr = str.substring(0, length / 2);
        String rightStr = str.substring(length / 2, length);
        return (reverse(rightStr) + reverse(leftStr));
    }
}

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