32. 最长有效括号

栈

class Solution {
    public int longestValidParentheses(String s) {
        Deque<Integer> stack = new ArrayDeque<>();
        stack.push(-1);
        int res = 0;

        for(int i = 0; i < s.length(); i++){
            if(s.charAt(i) == '('){
                stack.push(i);
            }else{
                stack.pop();
                if(stack.isEmpty()){
                    stack.push(i);
                }else{
                    res = Math.max(res, i - stack.peek());
                }
            }
        }

        return res;
    }
}