[leetcode]32. Longest Valid Parentheses最长合法括号子串

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

Example 1:

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

Example 2:

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

题意：

给定一个括号序列，找出最长的合法括号子串。

Solution1:  Two pointers(fast&slow)  + Stack

It's just like using [start+1...i] to maintain a window where contains longest valid parentheses

code:

/*
Time: O(n). We traverse the given input
Space: O(n). Worse case, input is all contained '（'， then there will be n chars stored in the stack
*/
class Solution {
    public int longestValidParentheses(String s) {
        int result = 0;
        int start = -1;
        Stack<Integer> stack = new Stack<>(); // index of every char
        
        for(int i = 0; i < s.length(); i++){
            char c = s.charAt(i);
            if(c == '('){
                stack.push(i);   
            }else if( c == ')'){
                if(stack.isEmpty()){
                    // no matching left '( '
                    start = i;
                }else{
                    stack.pop();
                    if(stack.isEmpty()){
                        result = Math.max(result, i - start);
                    }else{
                        result = Math.max(result, i - stack.peek());
                    }
                }
            }
        }
        return result;
    }
}