Reverse a String easy

Description

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.

Examples

> Case 1:
    Input: s = ["h","e","l","l","o"]
    Output: ["o","l","l","e","h"]
 
> Case 2:
    Input: s = ["H","a","n","n","a","h"]
    Output: ["h","a","n","n","a","H"]

Constraints

• 1 <= s.length <= 10^5
• s[i] is a printable ASCII character

Approach 1: Two Pointers (Optimal)

The classic two-pointer approach: one pointer starts at the beginning, the other at the end. Swap and move inward until they meet.

#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--;
    }
}

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
 
# Example
s = ["h", "e", "l", "l", "o"]
reverse_string(s)
print(s)  # ["o", "l", "l", "e", "h"]
 
# Python shortcut (but less educational):
# s.reverse() or s[:] = s[::-1]

class Solution {
    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--;
        }
    }
}

Explanation

• Initialize two pointers: left at position 0 and right at the last position
• While left < right, swap the characters at these two positions
• Move left forward and right backward
• When the pointers meet or cross, every character has been swapped and the string is reversed

Analysis

• Time Complexity: O(n) — each character is visited once
• Space Complexity: O(1) — only uses two pointer variables