LeetCode: Restore IP Addresses [093]

class Solution {
public:
    bool isValid(string num){
        if(num.length()==1 && num>="0" && num<="9")return true;
        if(num.length()==2 && num>="10" && num<="99")return true;
        if(num.length()==3 && num>="100" && num<="255")return true;
        return false;
    }

    void restoreIP(vector<string>&result, vector<string>&ips, int kth, int start, string s){
        if(start==s.length()){
            if(kth==5){
                //返回字符串
                string ip=ips[0];
                for(int i=1; i<4; i++)
                    ip+="."+ips[i];
                result.push_back(ip);
            }
            return;
        }
        
        //从s的start位置开始确定ip串中的第kth个数
        for(int k=1; k<=3 && start+(k-1)<s.length(); k++){
            //向前探k位数字
            string num=s.substr(start, k);
            if(isValid(num)){
                ips.push_back(num);
                restoreIP(result, ips, kth+1, start+k, s);
                ips.pop_back();
            }
        }
    }

    vector<string> restoreIpAddresses(string s) {
        vector<string> result;
        if(s.length()<4 || s.length()>12)return result; //注意字符串有效长度
        
        vector<string>ips;
        restoreIP(result, ips, 1, 0, s);
        return result;
    }
};