344. Reverse String

Difficulty: Easy

Topics: Two Pointers, 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.

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:

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

Hint:

The entire logic for reversing a string is based on using the opposite directional two-pointer approach!

Solution:

To solve this problem, we can follow these steps:

Let's implement this solution in PHP: 344. Reverse String

<?php
/**
 * @param String[] $s
 * @return NULL
 */
function reverseString(&$s) {
    ...
    ...
    ...
    /**
     * go to ./solution.php
     */
}

// Example 1:
$s1 = ["h", "e", "l", "l", "o"];
reverseString($s1);
print_r($s1); // Output: ["o", "l", "l", "e", "h"]

// Example 2:
$s2 = ["H", "a", "n", "n", "a", "h"];
reverseString($s2);
print_r($s2); // Output: ["h", "a", "n", "n", "a", "H"]
?>

Explanation:

Initialization:
- Set two pointers: left starting from the beginning (0) and right from the end (count($s) - 1).
Swapping:
- Swap the elements at left and right positions.
- Increment the left pointer and decrement the right pointer after each swap.
Loop:
- Continue the swapping until left is less than right.
In-Place Modification:
- The input array $s is modified in place, meaning the original array is reversed without using any additional arrays.

This solution has a time complexity of O(n) and a space complexity of O(1), as required.

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