NTL_1_B: Power

問題へのリンク

C#のソース

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

// Q084 ModPow https://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=NTL_1_B&lang=jp
class Program
{
    static string InputPattern = "InputX";

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

        if (InputPattern == "Input1") {
            WillReturn.Add("2 3");
            //8
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("5 8");
            //390625
        }
        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 m = wkArr[0];
        long n = wkArr[1];

        const long Hou = 1000000007;

        long Result = DeriveModPow(m, n, Hou);
        Console.WriteLine(Result);
    }

    //繰り返し2乗法で、(NのP乗) Mod Mを求める
    static long DeriveModPow(long pN, long pP, long pM)
    {
        long CurrJyousuu = pN % pM;
        long CurrShisuu = 1;
        long WillReturn = 1;

        while (true) {
            //対象ビットが立っている場合
            if ((pP & CurrShisuu) > 0) {
                WillReturn = (WillReturn * CurrJyousuu) % pM;
            }

            CurrShisuu *= 2;
            if (CurrShisuu > pP) return WillReturn;
            CurrJyousuu = (CurrJyousuu * CurrJyousuu) % pM;
        }
    }
}

解説

繰り返し2乗法を使ってます。