LeetCode——032

/*
32. Longest Valid Parentheses My Submissions QuestionEditorial Solution
Total Accepted: 60805 Total Submissions: 271732 Difficulty: Hard
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.

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*/

/*
解题思路：
这道求最长有效括号比之前那道 Valid Parentheses 验证括号难度要大一些，这里我们还是借助栈来求解，需要定义个start变量来记录合法括号串的起始位置，我们遍历字符串，如果遇到左括号，则将当前下标压入栈，如果遇到右括号，如果当前栈为空，则将下一个坐标位置记录到start，如果栈不为空，则将栈顶元素取出，此时若栈为空，则更新结果和i - start + 1中的较大值，否则更新结果和i - 栈顶元素中的较大值，代码如下：
*/

class Solution {
public:
    int longestValidParentheses(string s) {
        int res = 0, start = 0;
        stack<int> m;
        for (int i = 0; i < s.size(); ++i) {
            if (s[i] == '(') m.push(i);
            else if (s[i] == ')') {
                if (m.empty()) start = i + 1;
                else {
                    m.pop();
                    res = m.empty() ? max(res, i - start + 1) : max(res, i - m.top());
                }
            }
        }
        return res;
    }
};