LeetCode: 344. Reverse String-简单-Python+Java

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"]

 

解题思路：

用两个指针，分别指向头和尾，互相交换值（java需要一个temp变量来保存值 O(1)），头指针向后移动，尾指针向前移动，直到头指针大于等于尾部指针

时间 o（n/2） 空间o（1）

 

Java:

class Solution {
    public void reverseString(char[] s) {
        int start = 0;
        int end = s.length-1;
        while(start<end){
            char tmp = s[end];
            s[end--] = s[start];
            s[start++] = tmp;
        }
    }
}

 Python：

class Solution(object):
    def reverseString(self, s):
        """
        :type s: List[str]
        :rtype: None Do not return anything, modify s in-place instead.
        """
        start, end = 0, len(s)-1
        while start < end:
            s[start], s[end] = s[end],s[start];
            start += 1
            end -= 1