344. Reverse String

Problem

Write a function that reverses a string. The input string is given as an array of characters s.

You must do this by modifying the input array in-place with O(1) extra memory.

Example 1:

Input: s = [“h”,“e”,“l”,“l”,“o”] Output: [“o”,“l”,“l”,“e”,“h”]

Example 2:

Input: s = [“H”,“a”,“n”,“n”,“a”,“h”] Output: [“h”,“a”,“n”,“n”,“a”,“H”]

Constraints:

• 1 <= s.length <= 105
• s[i] is a printable ascii character.

Solution

Look at Swapping to cover other methods to swap elements other than a temp variable;

Two Pointer - O(n) time - O(1) space

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

Recursion - O(n) time - O(1) space

class Solution {
  public void helper(char[] s, int left, int right) {
    if (left >= right) return;
    char tmp = s[left];
    s[left++] = s[right];
    s[right--] = tmp;
    helper(s, left, right);
  }
 
  public void reverseString(char[] s) {
    helper(s, 0, s.length - 1);
  }
}