Leetcode: Longest Valid Parentheses

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.

思路

1. 状态模拟题, 最不喜欢的题目

2. 参考别人的思路, o(n) 完美解决, 漂亮的代码

3. stack 并不存字符, 而是存储左括号的位置, 失去匹配的右括号作为分隔符

代码

class Solution {
public:
	int ans;
	int sum;
    int longestValidParentheses(string s) {
		ans = sum = 0;
		deque<int> stack;
		if(s.size() <= 0)
			return 0;
		int last = -1;
		for(int i = 0; i < s.size(); i ++) {
			if(s[i] == '(') {
				stack.push_back(i);
			}else{
				if(stack.empty()) {
					last = i;
				}else{
					stack.pop_back();
					if(stack.empty()) {
						ans = max(ans, i-last);
					}else{
						ans = max(ans, i-stack.back());
					}
					
				}
			}
		}
		return ans;
	}
};