典型90問 069 Colorful Blocks 2（★3）

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("2 3");
            //6
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("10 2");
            //0
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("2021 617");
            //53731843
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    const long Hou = 1000000007;

    static void Main()
    {
        List<string> InputList = GetInputList();
        long[] wkArr = InputList[0].Split(' ').Select(pX => long.Parse(pX)).ToArray();
        long N = wkArr[0];
        long K = wkArr[1];

        long Answer = 1;

        if (N == 1) {
            Answer *= K;
            Answer %= Hou;
            Console.WriteLine(Answer);
            return;
        }
        if (N == 2) {
            Answer *= K;
            Answer %= Hou;
            Answer *= K - 1;
            Answer %= Hou;
            Console.WriteLine(Answer);
            return;
        }

        Answer *= K;
        Answer %= Hou;
        Answer *= K - 1;
        Answer %= Hou;
        Answer *= K - 2;
        Answer %= Hou;

        long RestN = N - 3;
        if (RestN >= 1) {
            Answer *= DeriveBekijyou(K - 2, RestN, Hou);
            Answer %= Hou;
        }
        Console.WriteLine(Answer);
    }

    // 繰り返し2乗法で、(NのP乗) Mod Mを求める
    static long DeriveBekijyou(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

設置する個数で場合分けしてます。