CANDY3.cs

using System;

// https://www.spoj.com/problems/CANDY3/ #division #mod-math
// Determines if N bags full of candies can have their contents
// redistributed equally amongst N children.
public static class CANDY3
{
    // I guess the numbers will be too big if summed directly, so need to use
    // property of modular arithmetic: (a + b) mod m == (a mod m + b mod m) mod m
    public static bool Solve(ulong[] backpackCandyCounts)
    {
        uint N = (uint)backpackCandyCounts.Length;
        ulong modSum = 0;

        for (int i = 0; i < N; ++i)
        {
            modSum = (modSum + (backpackCandyCounts[i] % N)) % N;
        }

        return modSum == 0;
    }
}

public static class Program
{
    private static void Main()
    {
        int remainingTestCases = int.Parse(Console.ReadLine());
        while (remainingTestCases-- > 0)
        {
            Console.ReadLine();

            int backpackCount = int.Parse(Console.ReadLine());
            ulong[] backpackCandyCounts = new ulong[backpackCount];
            for (int i = 0; i < backpackCount; ++i)
            {
                backpackCandyCounts[i] = ulong.Parse(Console.ReadLine());
            }

            Console.WriteLine(
                CANDY3.Solve(backpackCandyCounts) ? "YES" : "NO");
        }
    }
}