题目：

题解：

int longestValidParentheses(char* s) {
    int n = strlen(s);
    int left = 0, right = 0, maxlength = 0;
    for (int i = 0; i < n; i++) {
        if (s[i] == '(') {
            left++;
        } else {
            right++;
        }
        if (left == right) {
            maxlength = fmax(maxlength, 2 * right);
        } else if (right > left) {
            left = right = 0;
        }
    }
    left = right = 0;
    for (int i = n - 1; i >= 0; i--) {
        if (s[i] == '(') {
            left++;
        } else {
            right++;
        }
        if (left == right) {
            maxlength = fmax(maxlength, 2 * left);
        } else if (left > right) {
            left = right = 0;
        }
    }
    return maxlength;
}