Reverse String
(#344), Easy
Category: String, Two Pointer
Problem Statement
Write a function that reverses a character array in-place. You must do this by modifying the input array directly with O(1) extra memory.
Examples
Input: ["h","e","l","l","o"]
Output: ["o","l","l","e","h"]
Input: ["H","a","n","n","a","h"]
Output: ["h","a","n","n","a","H"]
Approach
Two Pointer In-place Reversal
- Initialize two pointers: `left = 0` and `right = s.length - 1`.
- While `left < right`, swap `s[left]` and `s[right]`.
- Increment `left` and decrement `right` after each swap.
- This continues until the pointers meet in the middle.
- Note: This approach modifies the array in-place and uses no extra memory.
// 🔹 Two Pointer In-place Reversal
var reverseString = function(s) {
let left = 0, right = s.length - 1;
while (left < right) {
[s[left], s[right]] = [s[right], s[left]]; // Swap using destructuring
left++;
right--;
}
};Complexity
Time: O(n)
Space: O(1)