[LeetCode] Longest Valid Parentheses 最长有效括号

 

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.

 

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;
    }
};

 

还有一种利用动态规划Dynamic Programming的解法,可参见网友喜刷刷的博客

 

类似题目:

Remove Invalid Parentheses 移除非法括号

Different Ways to Add Parentheses

Generate Parentheses

Valid Parentheses

 

    原文作者:Grandyang
    原文地址: http://www.cnblogs.com/grandyang/p/4424731.html
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