程序媛詹妮弗
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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:

 1 /*
 2 Time: O(n). We traverse the given input
 3 Space: O(n). Worse case, input is all contained '(', then there will be n chars stored in the stack
 4 */
 5 class Solution {
 6     public int longestValidParentheses(String s) {
 7         int result = 0;
 8         int start = -1;
 9         Stack<Integer> stack = new Stack<>(); // index of every char
10         
11         for(int i = 0; i < s.length(); i++){
12             char c = s.charAt(i);
13             if(c == '('){
14                 stack.push(i);   
15             }else if( c == ')'){
16                 if(stack.isEmpty()){
17                     // no matching left '( '
18                     start = i;
19                 }else{
20                     stack.pop();
21                     if(stack.isEmpty()){
22                         result = Math.max(result, i - start);
23                     }else{
24                         result = Math.max(result, i - stack.peek());
25                     }
26                 }
27             }
28         }
29         return result;
30     }
31 }

 

posted on 2018-06-21 06:58  程序媛詹妮弗  阅读(176)  评论(0编辑  收藏  举报