Longest Valid Parentheses

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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：使用vector類比棧，棧頂元素表示出現在當前‘(’之後的合法匹配數，棧本身具有一個哨位節點。依次遍曆s，若當前字元s[i]==‘(‘，則將0入棧；否則，將棧頂元素加2後再加到次棧頂元素，同時棧頂元素出棧。

 1 class Solution { 2 public: 3     int longestValidParentheses( string s ) { 4         int slen = s.length(), idx = 0, ret = 0; 5         if( slen <= 1 ) { return 0; } 6         vector<int> cntVec( slen+1, 0 ); 7         for( int i = 0; i < slen; ++i ) { 8             if( s[i] == ‘(‘ ) { 9                 cntVec[++idx] = 0;10             } else {11                 if( idx > 0 ) {12                     cntVec[idx-1] += cntVec[idx]+2;13                     ret = max( ret, cntVec[--idx] );14                 } else {15                     cntVec[0] = 0;16                 }17             }18         }19         return ret;20     }21 };

 

Longest Valid Parentheses