4. Reverse String

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.

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

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

Problem Breakdown:

1. We use a two-pointer approach. We initialize a left pointer at the beginning (index 0) and a right pointer at the end (last index) of the array.

• left = 0
  right = len(s) - 1

2. We loop while the left pointer is less than the right pointer. This ensures we process each pair of characters only once and stop when the pointers meet in the middle.

• while left < right:

3. Inside the loop, we swap the characters at the left and right positions. Python allows us to do this elegantly in one line using tuple unpacking.

• s[left], s[right] = s[right], s[left]

4. After the swap, we move both pointers inward - increment left and decrement right - to process the next pair of characters.

• left += 1
  right -= 1

Summary:

Use two pointers starting from opposite ends of the array. Swap the characters at each pointer, then move both pointers toward the center. This reverses the array in-place without using extra space. The process continues until the pointers meet in the middle.

Time and Space Complexity:

Time Complexity: O(n) - we process each character once, where n is the length of the string.

Space Complexity: O(1) - we only use two pointer variables regardless of input size.

Python Solution:

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

JavaScript Solution:

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

Java Solution:

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