Reverse String

Description

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"]

Code

Iteration

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

Recursion

public String reverseString(String s) {
    int len = s.length();
    if (len <= 1) return s;
    String leftStr = s.substring(0, len / 2);
    String rightStr = s.substring(len / 2, len);
    return reverseString(rightStr) + reverseString(leftStr);
}