leetcode 344. Reverse String

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 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"]

难度：简单题

题目大意：反转字符串

思路：双指针、递归

方法一：递归

C++代码：

class Solution {
public:
    void dfs(vector<char> &s, int l, int r) {
        if (l >= r)
            return;
        swap(s[l], s[r]);
        dfs(s, l + 1, r - 1);
    }
    void reverseString(vector<char>& s) {
        dfs(s, 0, s.size() - 1);
    }
};

时间复杂度：$O(n)$

空间复杂度：$O(n)$(递归栈）

python3代码：

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

---

方法二：双指针

C++代码：

class Solution {
public:
    void reverseString(vector<char>& s) {
        for(int i = 0, j = s.size() - 1; i < j; ++i,--j) {
            swap(s[i], s[j]); //辅助变量实现、异或运算实现
        }
    }
};

python3代码：

class Solution:
    def reverseString(self, s: List[str]) -> None:
        """
        Do not return anything, modify s in-place instead.
        """
        # s.reverse()
        # for i in range(len(s) // 2):
        #     s[i], s[-1-i] = s[-1-i], s[i]
        i, j = 0, len(s) - 1
        while i < j:
            s[i], s[j] = s[j], s[i]
            i, j = i + 1, j - 1