344 Reverse String 反转字符串

Description:
Write a function that reverses a string. The input string is given as an array of characters char[].

Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory.

You may assume all the characters consist of printable ascii characters.

Example :

Example 1:
Input: ["h","e","l","l","o"]
Output: ["o","l","l","e","h"]

Example 2:
Input: ["H","a","n","n","a","h"]
Output: ["h","a","n","n","a","H"]

题目描述:
编写一个函数，其作用是将输入的字符串反转过来。输入字符串以字符数组 char[] 的形式给出。

不要给另外的数组分配额外的空间，你必须原地修改输入数组、使用 O(1) 的额外空间解决这一问题。

你可以假设数组中的所有字符都是 ASCII 码表中的可打印字符。

示例 :

示例 1：
输入：["h","e","l","l","o"]
输出：["o","l","l","e","h"]

示例 2：
输入：["H","a","n","n","a","h"]
输出：["h","a","n","n","a","H"]

思路:

1. 双指针循环交换, 注意到交换的都是可打印字符串, 交换可以用异或进行
2. 递归
   时间复杂度O(n), 空间复杂度O(1)

代码:
C++:

class Solution 
{
public:
    void reverseString(vector<char>& s) 
    {
        int i = 0, j = s.size() - 1;
        while (i < j) 
        {
            s[i] ^= s[j];
            s[j] ^= s[i];
            s[i++] ^= s[j--];
        }
    }
};

Java:

class Solution {
    public void reverseString(char[] s) {
        swap(0, s.length - 1, s);
    }

    public void swap(int start, int end, char[] s) {
        if (start >= end) {
            return;
        }
        char temp = s[start];
        s[start] = s[end];
        s[end] = temp;
        swap(start + 1, end - 1, s);
    }
}

Python:

class Solution:
    def reverseString(self, s: List[str]) -> None:
        """
        Do not return anything, modify s in-place instead.
        """
        s.reverse()