Leetcode 301 - remove invalid parenthesis -

Leetcode301_removeInvalideParentheses.cpp

class Solution {  
public:  
    void DFS(string s, char ch, int last)  
    {  
        for(int i = 0, cnt = 0; i < s.size(); i++)  
        {  
            if(s[i]=='('||s[i]==')') s[i]==ch?cnt++:cnt--;  
            if(cnt <= 0) continue;  
            for(int j = last; j <= i; j++)  
            {  
                if(s[j] == ch && (j ==last || s[j-1]!= ch))  
                    DFS(s.substr(0, j)+s.substr(j+1), ch, j);  
            }  
            return;  
        }  
        reverse(s.begin(), s.end());  
        if(ch == ')') return DFS(s, '(', 0);  
        ans.push_back(s);  
    }  
      
    vector<string> removeInvalidParentheses(string s) {  
        DFS(s, ')', 0);  
        return ans;  
    }  
private:  
    vector<string> ans;  
};