A40 Triangle

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("7");
            WillReturn.Add("1 2 1 2 1 2 1");
            //5
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

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

        var CntDict = new Dictionary<long, long>();
        foreach (long EachA in AArr) {
            if (CntDict.ContainsKey(EachA) == false) {
                CntDict[EachA] = 0;
            }
            CntDict[EachA]++;
        }

        long Answer = 0;
        foreach (long EachVal in CntDict.Values) {
            if (EachVal < 3) continue;

            Answer += EachVal * (EachVal - 1) * (EachVal - 2) / 6;
        }
        Console.WriteLine(Answer);
    }
}

解説

度数分布表を求めてから、Chooseの公式を使ってます。