344.反转字符串

使用双指针：

class Solution {
    public void reverseString(char[] s) {
        int l = 0,r = s.length-1;
        while(l<r){
            char c = s[l];
            s[l] = s[r];
            s[r] = c; 
            l++;
            r--;
        }
    }
}