LeetCode:Longest Valid Parentheses

标签：leetcode   dp   

           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.

解题思路:

    这题可以用栈或者dp做，不过自己用栈写的O(N)的解法没有dp的快，所以说下dp的思路吧.

首先，看下状态的定义:

• dp[i]:表示选了第i个字符能组成的最长有效括号个数.

class Solution {
public:
    int longestValidParentheses(string s) 
    {
        int n = s.size(), dp[n];
        dp[0] = 0;
        for (int i = 1; i < n; ++i)
        {
            int tmp = i - 1 - dp[i - 1];
            if (s[i] == '(')
                dp[i] = 0;
            else if (tmp >= 0 && s[tmp] == '(')
                dp[i] = dp[i - 1] + 2 + (tmp - 1 >= 0 ? dp[tmp - 1] : 0);
            else
                dp[i] = 0;
        }
        return *max_element(dp, dp + n);
    }
};

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LeetCode:Longest Valid Parentheses

标签：leetcode   dp   

原文地址：http://blog.csdn.net/dream_you_to_life/article/details/36063231