[LeetCode]Longest Valid Parentheses

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.

思考：无匹配的‘）’为断点。

class Solution {
public:
int longestValidParentheses(string s) {
// IMPORTANT: Please reset any member data you declared, as
// the same Solution instance will be reused for each test case.
int ret=0,last=-1;
stack<int> left;
for(int i=0;i<s.size();i++)
{
if(s[i]=='(') left.push(i);
else  //s[i]==')'
{
if(left.empty()) last=i;
else
{
left.pop();
if(left.empty()) ret=max(ret,i-last);
else ret=max(ret,i-left.top());
}
}
}
return ret;
}
};