leetcode Longest Valid Parentheses

Longest Valid Parentheses

  Total Accepted: 4153  Total Submissions: 22587 My Submissions

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.

每次操作栈和其他容器的时候都要注意是否为空。

这道题的计算要注意，如何才能得到当前最长valid parentheses。

int longestValidParentheses(string s) 
	{
		stack<int> stk;
		int len = 0;

		for (int i = 0; i < s.length(); i++)
		{
			if (s[i] == '(') stk.push(i);
			else
			{
				if (stk.empty()) stk.push(i);
				else
				{
					if (s[stk.top()] == '(')
					{
						stk.pop();
						if (stk.empty()) len = max(len, i+1);
						else len = max(len, i-stk.top());
					}
					else stk.push(i);
				}
			}
		}
		return len;
	}

巧妙地利用栈，保留下标，然后根据条件判断，逐步计算出结果。

分支条件有三到四个，也挺容易出错的题目。

 class Solution {
 public:
     int longestValidParentheses(string s)
     {
         int n = s.length();
         stack<int> stk;
         int longestLen = 0;
         for (int i = 0; i < n; i++)
         {
             if (s[i] == '(') stk.push(i);
             else
             {
                 if (stk.empty())
                 {
                     stk.push(i);
                 }
                 else
                 {
                     if (s[stk.top()] != '(')
                         stk.push(i);
                     else
                     {
                         stk.pop();
                         //居然会漏了max写成maxLen = (maxLen, i+1)
                         if (stk.empty()) longestLen = max(longestLen, i+1);
                         else    longestLen = max(longestLen, i-stk.top());
                     }
                 }
             }
         }
         return longestLen;
     }
 };