题目
Given a string containing just the characters '(' and ')', find the length of the longest valid (wellformed) 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.
class Solution {
public:
int longestValidParentheses(const string& s) {
int max_len = 0, last = -1; // the position of the last ')'
stack<int> lefts; // keep track of the positions of non-matching '('s
for (int i = 0; i < s.size(); ++i) {
if (s[i] =='(') {
lefts.push(i);
} else {
if (lefts.empty()) {
// no matching left
last = i;
} else {
// find a matching pair
lefts.pop();
if (lefts.empty()) {
max_len = max(max_len, i-last);
} else {
max_len = max(max_len, i-lefts.top());
}
}
}
}
return max_len;
}
};
