【LeetCode】Longest Valid Parentheses

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Longest Valid Parentheses

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.

 

括弧匹配的常規思路就是用進出棧。

但是這題的關鍵在於“連續匹配”，因此計算“連續匹配”長度時，本質就是求不匹配的括弧之間最大長度。

也就是求進棧但未出棧的括弧下標之間的最大差值，注意邊界值：第一個留在棧中的括弧之前的連續匹配長度。

 

class Solution {public:    int longestValidParentheses(string s) {        int ret = 0;        stack<int> stk;   //store the indexes of unmatched parentheses        for(int i = 0; i < s.size(); i ++)        {            if(s[i] == ‘(‘)            //unmatch                stk.push(i);            else            {//s[i] == ‘)‘                if(!stk.empty() && s[stk.top()] == ‘(‘)                //match                    stk.pop();                else                //unmatch                    stk.push(i);            }        }        if(stk.empty())        //all match            ret = s.size();        else        {//check every unmatched pair of parentheses            int start;            int end = s.size()-1;            while(!stk.empty())            {                start = stk.top();                stk.pop();                ret = max(ret, end-start);                end = start-1;            }            //from begin to the first unmatched parenthese            ret = max(ret, end+1);        }        return ret;    }};

【LeetCode】Longest Valid Parentheses