leetcode栈--3、longest-valid-parentheses(最长有效匹配括号长度)

题目描述
 
Given a string containing just the characters'('and')', find the length of the longest valid (well-formed) parentheses substring.
For"(()", the longest valid parentheses substring is"()", which has length = 2.
Another example is")()())", where the longest valid parentheses substring is"()()", which has length = 4.
 
解题思路:这里我们还是借助栈来求解,需要定义个start变量来记录合法括号串的起始位置,我们遍历字符串,如果遇到左括号,则将当前下标压入栈,如果遇到右括号,如果当前栈为空,则将下一个坐标位置记录到start,如果栈不为空,则将栈顶元素取出,此时若栈为空,则更新最长长度,为之前的len和i - start + 1中的较大值,否则更新长度为len和i - 栈顶元素中的较大值
 1 class Solution {
 2 public:
 3     int longestValidParentheses(string s) {
 4         int len = 0;
 5         int start = 0;
 6         int n = s.size();
 7         stack<int> ss;
 8         for(int i=0;i<n;i++)
 9         {
10             if(s[i]=='(')
11             {
12                 ss.push(i);
13             }
14             else
15             {
16                 if(!ss.empty())
17                 {
18                     ss.pop();
19                     len = ss.empty() ? max(len, i - start + 1) : max(len, i - ss.top());;
20                 }
21                 else
22                 {
23                     start = i+1;
24                 }
25             }
26         }
27         return len;
28     }
29 };

 

posted @ 2017-06-05 10:41  qqky  阅读(223)  评论(0编辑  收藏  举报