Reverse String

LeetCode 344 | Difficulty: Easy

Easy

Problem 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.

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:

- `1 <= s.length <= 10^5`

- `s[i]` is a [printable ascii character](https://en.wikipedia.org/wiki/ASCII#Printable_characters).

Topics: Two Pointers, String

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Approach

Direct Approach

This problem can typically be solved with straightforward iteration or simple data structure usage. Focus on correctness first, then optimize.

When to use

Basic problems that test fundamental programming skills.

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Solutions

Solution 1: C# (Best: 180 ms)

Metric	Value
Runtime	180 ms
Memory	N/A
Date	2018-04-04

Solution

public class Solution {
    public string ReverseString(string s) {
        char[] ar = s.ToCharArray();
        int i=0, j=s.Length-1;
        while(i<j)
        {
            char temp = ar[i];
            ar[i] = ar[j];
            ar[j] = temp;
            i++;
            j--;
        }
        return new string(ar);
    }
}

📜 2 more C# submission(s)

Submission (2022-04-01) — 360 ms, 47.9 MB

public class Solution {
    public void ReverseString(char[] s) {
        int i=0, j = s.Length-1;
        while(i<j)
        {
            char temp = s[i];
            s[i] = s[j];
            s[j] = temp;
            i++;
            j--;
        }
    }
}

Submission (2019-12-15) — 404 ms, 35.9 MB

public class Solution {
    public void ReverseString(char[] s) {
        helper(0, s.Length-1, s);
    }
    
    public void helper(int i, int j, char[] s)
    {
        if(i>=j) return;
        helper(i+1, j-1, s);
        char temp = s[i];
        s[i] = s[j];
        s[j] = temp;
    }
    
}

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Complexity Analysis

Approach	Time	Space
Two Pointers	$O(n)$	$O(1)$

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Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.
• LeetCode provides 1 hint(s) for this problem — try solving without them first.

💡 Hints

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