344. Reverse String

[mah-shamim]

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:

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

[mah-shamim]

To solve this problem, we can follow these steps:

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

<?php
function reverseString(&$s) {
    $left = 0;
    $right = count($s) - 1;

    while ($left < $right) {
        // Swap the characters
        $temp = $s[$left];
        $s[$left] = $s[$right];
        $s[$right] = $temp;

        // Move the pointers
        $left++;
        $right--;
    }
}

// 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:

1. Initialization:

   • Set two pointers: left starting from the beginning (0) and right from the end (count($s) - 1).

2. Swapping:

   • Swap the elements at left and right positions.
   • Increment the left pointer and decrement the right pointer after each swap.

3. Loop:

   • Continue the swapping until left is less than right.

4. 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.

[basharul-siddike]

Thanks to reverse a string using the two-pointer approach

[mah-shamim]

Thnaks