ABC174-C Repsept

問題へのリンク

C#のソース

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("101");
            //4
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("2");
            //-1
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("999983");
            //999982
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();

        long[] wkArr = InputList[0].Split(' ').Select(pX => long.Parse(pX)).ToArray();
        long K = long.Parse(InputList[0]);

        // 初項は7で、漸化式は10倍して7を足す
        long CurrVal = 7 % K;

        long MaxKouNo = 1;
        var AppearValSet = new HashSet<long>();
        while (true) {
            if (CurrVal == 0) {
                Console.WriteLine(MaxKouNo);
                return;
            }
            CurrVal = CurrVal * 10 + 7;
            CurrVal %= K;
            if (AppearValSet.Add(CurrVal) == false) {
                break;
            }
            MaxKouNo++;
        }
        Console.WriteLine(-1);
    }
}

解説

漸化式を求めてから、mod Kで考えてます。