设置头尾两个指针，依靠中间变量temp交换头尾指针所指元素，头指针后移，尾指针前移，直到头尾指针重合或者头指针在尾指针后面一个元素

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

class Solution(object):
    def reverseString(self, s):
        """
        :type s: List[str]
        :rtype: None Do not return anything, modify s in-place instead.
        """
        return s.reverse()