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"]

很簡單的一道題，但是有個陷阱：不能把swap單獨作為一個方法，因為這樣的話原來的array裏面並沒有實現元素交換。

class Solution {
    public void reverseString(char[] s) {
        int num = s.length;
        
 
if(s == null || num == 0) return;
        for(int i=0; i<num/2; i++) {
            char t;
            t = s[i];
            s[i] = s[num-1-i];
            s[num-1-i] = t;
        }
    }
}

leetcode 344. Reverse String