[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;
     }
 }