LeetCode 93. 复原IP地址

class Solution {
public:
    vector<string> res;
    void dfs(string s,string path,int idx,int cnt)//枚举下一个逗号填哪
    {
        if(idx>=s.size())
        {
            if(idx==s.size()&&cnt==4)
            {
                path=path.substr(0,path.size()-1);
                res.push_back(path);
            }  
            return;
        }
        if(cnt>3)   return;
        for(int i=1;i<=3;i++)
        {
            string t=s.substr(idx,i);
            int num=stoi(t);
            if(num>=0&&num<=255)
            {
                if(t.size()>1&&t[0]=='0')  continue;
                else
                    dfs(s,path+t+'.',idx+i,cnt+1);
            }
        }
    }
    vector<string> restoreIpAddresses(string s) {
        dfs(s,"",0,0);
        return res;
    }
};