32. Longest Valid Parentheses

Given a string containing just the characters '(' and ')', return the length of the longest valid (well-formed) parentheses substring.

Example 1:

Input: s = "(()"
Output: 2
Explanation: The longest valid parentheses substring is "()".

Example 2:

Input: s = ")()())"
Output: 4
Explanation: The longest valid parentheses substring is "()()".

Example 3:

Input: s = ""
Output: 0

public int longestValidParentheses(String s) {
    int max = 0, start = 0;
    if(null == s) return 0;

    int len = s.length();

    Stack<Integer> stack = new Stack<>();
    for(int index = 0; index < len; index++){
        //遇左括号(，压栈(栈中元素为当前位置所处的下标)
        if('(' == s.charAt(index)){
            stack.push(index);
            continue;
        } else {
            if(stack.isEmpty()){
                start = index+1;
                continue;
            } else {
                stack.pop();
                if(stack.isEmpty()){
                    max = Math.max(max, index-start+1);
                } else {
                    max = Math.max(max, index-stack.peek());
                }
            }
        }
    }

    return max;
}