344. Reverse String Easy

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"]

Approach

Input: A character list s, representing the original string. It requires an in-place reverse.

Output: Reverse the order of characters, modifying the input list s directly without returning a new value.

This is a classic opposite-direction two pointers problem.

We can define two pointers: left = 0, right = len(s) - 1, pointing to the start and the end of the string respectively.

Then perform the following operations:

• While left < right, constantly swap s[left] and s[right];
• After each swap, move left one step to the right, and move right one step to the left;
• When the two pointers meet or cross, it implies the entire string has been completely reversed.

Implementation

python

class Solution:
    def reverseString(self, s: List[str]) -> None:
        """
        Do not return anything, modify s in-place instead.
        """
        left, right = 0, len(s) - 1

        # Swap left and right characters while the left pointer is less than the right pointer
        while left < right:
            # Use Python's swap syntax, omitting the temporary variable
            s[left], s[right] = s[right], s[left]

            # Pointers move towards each other
            left += 1
            right -= 1

javascript

/**
 * @param {character[]} s
 * @return {void} Do not return anything, modify s in-place instead.
 */
const reverseString = function(s) {
    let left = 0;
    let right = s.length - 1;

    while (left < right) {
        const temp = s[left];

        s[left] = s[right];
        s[right] = temp;

        left ++;
        right --;
    }
};

Complexity Analysis

• Time Complexity: O(n)
• Space Complexity: O(1)

Links

344. Reverse String (English)344. 反转字符串 (Chinese)