题目：

题解：

class Solution {
public:int longestValidParentheses(string s) {int left = 0, right = 0, maxlength = 0;for (int i = 0; i < s.length(); i++) {if (s[i] == '(') {left++;} else {right++;}if (left == right) {maxlength = max(maxlength, 2 * right);} else if (right > left) {left = right = 0;}}left = right = 0;for (int i = (int)s.length() - 1; i >= 0; i--) {if (s[i] == '(') {left++;} else {right++;}if (left == right) {maxlength = max(maxlength, 2 * left);} else if (left > right) {left = right = 0;}}return maxlength;}
};