Problem

Write a function that reverses a string. The input string is given as an array of characters. You must do this by modifying the input array in-place with O(1) extra memory.

Example

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

Solution

Use two pointers — one at the start, one at the end. Swap them and move both pointers toward the middle. Stop when they meet.

def reverse_string(s):
    left, right = 0, len(s) - 1
    while left < right:
        s[left], s[right] = s[right], s[left]
        left += 1
        right -= 1
    return s

function reverseString(s) {
    let left = 0;
    let right = s.length - 1;
    while (left < right) {
        [s[left], s[right]] = [s[right], s[left]];
        left++;
        right--;
    }
    return s;
}

#include <vector>
using namespace std;

void reverseString(vector<char>& s) {
    int left = 0, right = s.size() - 1;
    while (left < right) {
        swap(s[left], s[right]);
        left++;
        right--;
    }
}

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

Complexity

• Time: O(n) — we visit each character once
• Space: O(1) — only two pointer variables, no extra data structures

Why Two Pointers

The two-pointer technique is perfect when you need to process elements from both ends. It avoids creating a copy of the array (which would use O(n) extra space) and finishes in half the iterations of a naive approach.