344. Reverse String

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Two Pointers String Recursion

Time: O(n)

Space: O(n)

Intuition

Two-pointer swap: left starts at 0, right at n-1. Swap and move both inward until they meet.

Algorithm

1lo=0, hi=n-1. While lo < hi: swap(s[lo], s[hi]); lo++; hi--.

Example Walkthrough

Input: ["h","e","l","l","o"]

1.Swap h,o -> ["o","e","l","l","h"]. Swap e,l -> ["o","l","l","e","h"].

Output: ["o","l","l","e","h"]

Common Pitfalls

•In-place modification - no extra array needed.

344.cs

// Approach: Recursive two-pointer swap: exchange the outermost characters
// then recurse on the inner subarray.
// Time: O(n) Space: O(n)

public class Solution
{
    public void ReverseString(char[] s)
    {
        Reverse(0, s.Length - 1, s);
    }

    private void Reverse(int l, int r, char[] s)
    {
        if (l > r)
            return;

        char temp = s[l];
        s[l] = s[r];
        s[r] = temp;

        Reverse(l + 1, r - 1, s);
    }
}

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