[leetcode]Longest Valid Parentheses题解

题目：

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.

戴神版：

int longestValidParentheses(string s)
{
	int max_len= 0,last=-1;
	stack<int> l;
	for (int i=0;i<s.size();i++)
	{
		if (s[i]=='(')
		{
			l.push(i);
		} 
		else
		{
			if (l.empty())
			{
				last=i;	//?
			} 
			else
			{
				l.pop();
				if (l.empty())
				{
					max_len=max(max_len,i-last);
				} 
				else
				{
					max_len=max(max_len,i-l.top());
				}
			}
		}
	}
	return max_len;
}

我的思路：

1、用栈保存字符串中左括号的下标。

2、用数组保存字符串中字符是否合法的的下标。

3、统计连续合法的个数。

代码：

int longestValidParentheses(string s)
{
	int max_len= 0,count=0;
	stack<int> l;
	vector<bool> p(s.size());
	for (int i=0;i<s.size();i++)
	{
		if (s[i]=='(')
		{
			l.push(i);
		} 
		else if(!l.empty())
		{
			p[l.top()]=true;
			p[i]=true;
			l.pop();
		}
	}
	for (int i=0;i!=p.size();i++)
	{
		if (p[i])
		{
			count++;
		}
		else
		{
			max_len=max(max_len,count);
			count=0;
		}
	}
	max_len=max(max_len,count);
	return max_len;
}