32. Longest Valid Parentheses

Posted 2021-01-12 agentgamer

https://leetcode.com/problems/longest-valid-parentheses/description/

来回做了好几遍，虽然AC 了但是这个解发肯定不是最优的。思路是把问题分两步解决：1. 把字符串转换成一个匹配括号的id 序列，比如第0 个和第3 个匹配了，则把0，3都记录在一个数组里。2. 排序生成的这个数组，然后遍历一次找最长的连续串，比如1235 里面，123 就是最长的连续串。排序是必要的，考虑这个例子：()(())，生成的序列是012325, 排序以后则是12345，反应了匹配的括号是确实是连续的。

class Solution {
public:
    int longestValidParentheses(string s) {
        //transform the problem into 2 sub problems
        //1. convert string into match pairs
        //2. calculate connectivity from pairs
        
        if (s.length() < 2) {
            return 0;
        } 
        
        //converting matching pair
        vector<int> stack;
        vector<int> pairs;
        for (int i = 0; i < s.length(); i++) {
            if (s[i] == ‘(‘) {
                stack.push_back(i);
            }
            else if (stack.size() > 0) {
                auto top = *(stack.end()-1);
                stack.pop_back();
                pairs.push_back(top);
                pairs.push_back(i);
            }
        }
        
        //check conseqtivity
        std::sort(pairs.begin(), pairs.end());
        if (pairs.size() == 0) {
            return 0;
        }
        int last = pairs[0];
        int cur;
        int acc = 0;
        int m = 0;
        for (int i = 1; i < pairs.size(); i++) {
            cur = pairs[i];
            if (cur - last == 1) {
                acc++;
            } else {
                m = std::max(acc+1, m);
                acc = 0;
            }
            last = cur;
        }
        
        if (acc > 0) {
            m = std::max(acc+1, m);
        }
        
        return m;
    }
};