参考宫水三叶题解，非常有启发意义https://leetcode.cn/problems/valid-parenthesis-string/solutions/992601/gong-shui-san-xie-yi-ti-shuang-jie-dong-801rq/

32. 最长有效括号

使用栈+一次遍历，巧妙得出最长有效括号长度

func longestValidParentheses(s string) int {stack := make([]int,0)stack =append(stack,-1)mx := 0 for i:=0;i<len(s);i++{if s[i]=='('{stack = append(stack,i)}else{stack = stack[:len(stack)-1]if len(stack) == 0 {stack =append(stack,i)}else{mx  =max(mx,i-stack[len(stack)-1])}}}return mx
}