LeetCode Remove Invalid Parentheses

题意：

Remove the minimum number of invalid parentheses in order to make the input string valid. Return all possible results.

Note: The input string may contain letters other than the parentheses ( and ).

Examples:

"()())()" -> ["()()()", "(())()"]
"(a)())()" -> ["(a)()()", "(a())()"]
")(" -> [""]

思路： bfs， 每层修改一个字符，只要出现一个合法的，就停止构造，把队列中的处理完就ok了。

AC代码：

class Solution {
public:
    bool isvaild(string str)
    {
        int count = 0;
        for(int i=0; i<str.length(); i++)
        {
            if(str[i]!='(' && str[i]!=')')
                continue;
            if(str[i] == ')' && count == 0) return false;
            if(str[i] == '(') count+=1;
            else count -=  1;
        }
        return count == 0;
    }
    vector<string> removeInvalidParentheses(string s) {
        vector<string> ret;
        queue<string> q;
        set<string> st;
        q.push(s);
        st.insert(s);
        bool flag = false;
        while(!q.empty())
        {
            string str = q.front();
            q.pop();
            if(isvaild(str))
            {
                flag = true;
                ret.push_back(str);
            }
            if (flag) continue;
            for(int i=0; i<str.length(); i++)
            {
                if(str[i]!='(' && str[i]!=')')
                    continue;
                string ss = str.substr(0,i) + str.substr(i+1);
                if(st.count(ss) > 0) continue;
                st.insert(ss);
                q.push(ss);
            }
        }
        return ret;
    }
};