Write a function that takes a string as input and returns the string reversed.

Example:

题目大意就是给一个字符串，得到倒序字符串

corner case：空字符串或者字符串长度为0

解法一：

申请额外空间，index指向原字符串串尾，倒序构建新字符串

public class Solution {    public String reverseString(String s)     {        if (s == null || s.length() == 0)        {            return s;        }                String res = "";        int index = s.length() - 1;                for (; index >= 0; index--)        {            res += s.charAt(index);        }                return res;    }}

时间复杂度：n， 空间复杂度： n

解法二：

双pointer指向头尾，直接交换字符。不需要额外空间，只用循环n/2次

public class Solution {    public String reverseString(String s)     {        if (s == null || s.length() == 0)        {            return s;        }                int left = 0;        int right = s.length() - 1;        char[] sChar = s.toCharArray();                while (left < right)        {            char temp = sChar[left];            sChar[left] = sChar[right];            sChar[right] = temp;                        left++;            right--;        }                return new String(sChar);    }}