Problem:
Score : 400 points

Problem Statement

The ABC number of a string T is the number of triples of integers (i, j, k) that satisfy all of the following conditions:

1 ≤ i < j < k ≤ |T| (|T| is the length of T.)

T_i = A (T_i is the i-th character of T from the beginning.)

T_j = B

T_k = C

For example, when T = ABCBC, there are three triples of integers (i, j, k) that satisfy the conditions: (1, 2, 3), (1, 2, 5), (1, 4, 5). Thus, the ABC number of T is 3.

You are given a string S. Each character of S is A, B, C or ?.

Let Q be the number of occurrences of ? in S. We can make 3^Q strings by replacing each occurrence of ? in S with A, B or C. Find the sum of the ABC numbers of all these strings.

This sum can be extremely large, so print the sum modulo 10^9 + 7.

Constraints

3 ≤ |S| ≤ 10^5

Each character of S is A, B, C or ?.

Input

Input is given from Standard Input in the following format:

S

Output

Print the sum of the ABC numbers of all the 3^Q strings, modulo 10^9 + 7.

Sample Input 1

A??C

Sample Output 1

8

In this case, Q = 2, and we can make 3^Q = 9 strings by by replacing each occurrence of ? with A, B or C. The ABC number of each of these strings is as follows:

AAAC: 0

AABC: 2

AACC: 0

ABAC: 1

ABBC: 2

ABCC: 2

ACAC: 0

ACBC: 1

ACCC: 0

The sum of these is 0 + 2 + 0 + 1 + 2 + 2 + 0 + 1 + 0 = 8, so we print 8 modulo 10^9 + 7, that is, 8.

Sample Input 2

ABCBC

Sample Output 2

3

When Q = 0, we print the ABC number of S itself, modulo 10^9 + 7. This string is the same as the one given as an example in the problem statement, and its ABC number is 3.

Sample Input 3

????C?????B??????A???????

Sample Output 3

979596887

In this case, the sum of the ABC numbers of all the 3^Q strings is 2291979612924, and we should print this number modulo 10^9 + 7, that is, 979596887.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using static Ha2ne2Util.Cin;
using static Ha2ne2Util.Util;
using static System.Console;
using static System.Math;
using System.Collections;
using Pair = System.Tuple<long, long>;

/// <summary>
/// https://atcoder.jp/contests/abc104
/// 2020/02/02 3完 R1457
/// </summary>
namespace ABC104
{
    public class Program
    {
        public static void Main(string[] args)
        {
            // A(); 4m
            // B(); 29m
            // C(); 70m
            // D(); 降参

            // 2回目
            // A(); 1m
            // B(); 7m
            // C(); 28m
            D();
        }

        public static void D()
        {
            string S = ReadString();
            long[,] dp = new long[S.Length + 1, 4];
            dp[0, 0] = 1;

            for (int i = 0; i < S.Length; i++)
            {
                for (int j = 0; j < 4; j++)
                {
                    if (S[i] == '?')
                    {
                        dp[i + 1, j] = ModMul(dp[i, j], 3);
                    }
                    else
                    {
                        dp[i + 1, j] = dp[i, j];
                    }
                }

                if (S[i] == 'A' || S[i] == '?')
                    dp[i + 1, 1] = ModAdd(dp[i + 1, 1], dp[i, 0]);
                if (S[i] == 'B' || S[i] == '?')
                    dp[i + 1, 2] = ModAdd(dp[i + 1, 2], dp[i, 1]);
                if (S[i] == 'C' || S[i] == '?')
                    dp[i + 1, 3] = ModAdd(dp[i + 1, 3], dp[i, 2]);
            }

            Console.WriteLine(dp[S.Length, 3]);
        }

        public static void C()
        {
            int D = ReadInt();
            long G = rl;
            long[] P, C;
            ReadArrays(out P, out C, D);

            long ans = INF;

            int maxBit = 1 << D;
            for (int i = 0; i < maxBit; i++)
            {
                long score = 0;
                long count = 0;

                for (int j = 0; j < D; j++)
                {
                    if ((i & (1 << j)) == (1 << j))
                    {
                        score += (j + 1) * 100 * P[j] + C[j];
                        count += P[j];
                    }
                }

                if (score >= G)
                {
                    ChMin(ref ans, count);
                    continue;
                }

                int next = D - 1;
                while (next >= 0 && (i & (1 << next)) == (1 << next))
                {
                    next--;
                }
                if (next == -1) continue;

                var nokori = G - score;
                long problemScore = (next + 1) * 100;
                long maxScore = problemScore * P[next];
                if (maxScore < nokori)
                {
                    continue;
                }
                else
                {
                    count += nokori / problemScore;
                    if (nokori % problemScore != 0)
                        count++;

                    ChMin(ref ans, count);
                }
            }

            Console.WriteLine(ans);
        }

        public static void B()
        {
            string S = ReadString();

            string komoji = "abcdefghijklmnopqrstuvwxyz";
            HashSet<char> komojiSet = new HashSet<char>();
            foreach (var c in komoji)
            {
                komojiSet.Add(c);
            }

            if (S[0] != 'A')
            {
                Console.WriteLine("WA");
                return;
            }

            if (!komojiSet.Contains(S[1]))
            {
                Console.WriteLine("WA");
                return;
            }

            bool foundC = false;
            for (int i = 2; i < S.Length-1; i++)
            {
                if(S[i] == 'C' && foundC == false)
                {
                    foundC = true;
                }
                else if (komojiSet.Contains(S[i]))
                {
                    // nop
                }
                else
                {
                    Console.WriteLine("WA");
                    return;
                }
            }

            if (!foundC)
            {
                Console.WriteLine("WA");
                return;
            }

            if (!komojiSet.Contains(S[S.Length - 1]))
            {
                Console.WriteLine("WA");
                return;
            }

            Console.WriteLine("AC");
        }

        public static void A()
        {
            long R = rl;

            if (R < 1200)
            {
                Console.WriteLine("ABC");
            }
            else if (R < 2800)
            {
                Console.WriteLine("ARC");
            }
            else
            {
                Console.WriteLine("AGC");
            }
        }

        //public static void D()
        //{
        //    string S = ReadString();
        //    long[,] dp = new long[S.Length + 1, 4];
        //    dp[0, 0] = 1;

        //    for (int i = 0; i < S.Length; i++)
        //    {
        //        for (int j = 0; j < 4; j++)
        //        {
        //            if (S[i] == '?')
        //            {
        //                dp[i + 1, j] = ModMul(dp[i, j], 3);
        //            }
        //            else
        //            {
        //                dp[i + 1, j] = dp[i, j];
        //            }
        //        }

        //        if (S[i] == 'A' || S[i] == '?')
        //            dp[i + 1, 1] = ModAdd(dp[i + 1, 1], dp[i, 0]);
        //        if (S[i] == 'B' || S[i] == '?')
        //            dp[i + 1, 2] = ModAdd(dp[i + 1, 2], dp[i, 1]);
        //        if (S[i] == 'C' || S[i] == '?')
        //            dp[i + 1, 3] = ModAdd(dp[i + 1, 3], dp[i, 2]);
        //    }

        //    Console.WriteLine(dp[S.Length, 3]);
        //}

        //public static void C()
        //{
        //    int D = ReadInt();
        //    long G = rl;
        //    long[] P, C;
        //    ReadArrays(out P, out C, D);

        //    long ans = INF;

        //    int n = (1 << D) - 1;
        //    for (int i = n; i >= 0; i--)
        //    {
        //        long count = 0;
        //        long score = 0;

        //        for (int j = 0; j < D; j++)
        //        {
        //            if ((i & (1 << j)) == (1 << j))
        //            {
        //                count += P[j];
        //                score += (j + 1) * 100 * P[j] + C[j];
        //            }
        //        }

        //        if (score >= G)
        //        {
        //            ChMin(ref ans, count);
        //            continue;
        //        }

        //        long nokori = G - score;
        //        for (int j = D-1; j >= 0; j--)
        //        {
        //            if ((i & (1 << j)) == (1 << j))
        //                continue;

        //            var problemScore = (j + 1) * 100;
        //            var max = problemScore * P[j];
        //            if (nokori > max)
        //            {
        //                nokori -= max;
        //                count += P[j];
        //                continue;
        //            }

        //            count += nokori / problemScore;
        //            if (nokori % problemScore != 0)
        //                count++;
        //            break;                 
        //        }

        //        ChMin(ref ans, count);
        //    }

        //    Console.WriteLine(ans);
        //}

        //public static void B()
        //{
        //    // ^A、3～-2までにCが１個、ほかは小文字
        //    string S = ReadString();

        //    string komoji = "abcdefghijklmnopqrstuvwxyz";
        //    HashSet<Char> komojiSet = new HashSet<char>();
        //    foreach(var c in komoji)
        //    {
        //        komojiSet.Add(c);
        //    }

        //    if (S[0] != 'A')
        //    {
        //        Console.WriteLine("WA");
        //        return;
        //    }

        //    if (!komojiSet.Contains(S[1]))
        //    {
        //        Console.WriteLine("WA");
        //        return;
        //    }

        //    bool foundC = false;
        //    for (int i = 2; i <= S.Length - 2; i++)
        //    {
        //        if (S[i] == 'C' && foundC == false)
        //        {
        //            foundC = true;
        //        }
        //        else if (komojiSet.Contains(S[i]))
        //        {
        //            // nop
        //        }
        //        else
        //        {
        //            Console.WriteLine("WA");
        //            return;
        //        }
        //    }

        //    if (!foundC)
        //    {
        //        Console.WriteLine("WA");
        //        return;
        //    }

        //    if (!komojiSet.Contains(S[S.Length-1]))
        //    {
        //        Console.WriteLine("WA");
        //        return;
        //    }

        //    Console.WriteLine("AC");
        //}

        //public static void A()
        //{
        //    long R = rl;

        //    if (R < 1200)
        //    {
        //        Console.WriteLine("ABC");
        //    }
        //    else if (R < 2800)
        //    {
        //        Console.WriteLine("ARC");
        //    }
        //    else
        //    {
        //        Console.WriteLine("AGC");
        //    }
        //}
    }
}

namespace Ha2ne2Util
{
    public static class Cin
    {
        private static string[] Tokens { get; set; }

        private static int Pointer { get; set; }

        public static string Next()
        {
            if (Tokens == null || Tokens.Length <= Pointer)
            {
                Tokens = Console.ReadLine().Split(' ');
                Pointer = 0;
            }

            return Tokens[Pointer++];
        }

        public static int ReadInt()
        {
            return int.Parse(Next());
        }

        public static long rl => ReadLong();
        public static long[] rla => ReadLongArray();
        public static double rd => ReadDouble();
        public static double[] rda => ReadDoubleArray();

        public static long ReadLong()
        {
            return long.Parse(Next());
        }

        public static double ReadDouble()
        {
            return double.Parse(Next());
        }

        public static int[] ReadIntArray()
        {
            Tokens = null;
            return Array.ConvertAll(Console.ReadLine().Split(' '), int.Parse);
        }

        public static long[] ReadLongArray()
        {
            Tokens = null;
            return Array.ConvertAll(Console.ReadLine().Split(' '), long.Parse);
        }

        public static double[] ReadDoubleArray()
        {
            Tokens = null;
            return Array.ConvertAll(Console.ReadLine().Split(' '), double.Parse);
        }

        public static void ReadArrays(out long[] a, out long[] b, long N)
        {
            a = new long[N];
            b = new long[N];
            for (int i = 0; i < N; i++)
            {
                a[i] = ReadLong();
                b[i] = ReadLong();
            }
        }

        public static void ReadArrays(out long[] a, out long[] b, out long[] c, long N)
        {
            a = new long[N];
            b = new long[N];
            c = new long[N];
            for (int i = 0; i < N; i++)
            {
                a[i] = ReadLong();
                b[i] = ReadLong();
                c[i] = ReadLong();
            }
        }

        public static int[][] ReadIntTable(int n)
        {
            Tokens = null;
            int[][] ret = new int[n][];
            for (int i = 0; i < n; i++)
            {
                ret[i] = ReadIntArray();
            }

            return ret;
        }

        public static long[][] ReadLongTable(long n)
        {
            Tokens = null;
            long[][] ret = new long[n][];
            for (long i = 0; i < n; i++)
            {
                ret[i] = ReadLongArray();
            }

            return ret;
        }

        public static string ReadString()
        {
            return Next();
        }

        /// <summary>
        /// "00101001010"という様な入力をintの配列にして返す
        /// </summary>
        /// <returns></returns>
        public static int[] ReadIntArrayFromBinaryString()
        {
            Tokens = null;
            return Array.ConvertAll(Console.ReadLine().ToArray(), c => int.Parse(c.ToString()));
        }

        /// <summary>
        /// "00101001010"という様な入力をboolの配列にして返す
        /// </summary>
        /// <returns></returns>
        public static bool[] ReadBoolArrayFromBinaryString()
        {
            Tokens = null;
            return Console.ReadLine().Select(c => c == '1').ToArray();
        }
    }

    /// <summary>
    /// ユーティリティー
    /// </summary>
    public static class Util
    {
        // 10^17
        public static readonly long INF = (long)1e17;

        public static IEnumerable<int> Range(int end)
        {
            for (int i = 0; i < end; i++)
            {
                yield return i;
            }
        }

        public static IEnumerable<int> Range(int from, int end)
        {
            for (int i = from; i < end; i++)
            {
                yield return i;
            }
        }

        public static IEnumerable<int> Range(int from, int end, int step)
        {
            for (int i = from; i < end; i += step)
            {
                yield return i;
            }
        }

        /// <summary>
        /// ソートをして結果を返します。
        /// 破壊的関数です。
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="array"></param>
        /// <returns></returns>
        public static T[] Sort<T>(T[] array)
        {
            Array.Sort<T>(array);
            return array;
        }

        /// <summary>
        /// 降順ソートをして結果を返します。
        /// 破壊的関数です。
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="array"></param>
        /// <returns></returns>
        public static T[] SortDecend<T>(T[] array)
        {
            Array.Sort<T>(array);
            Array.Reverse(array);
            return array;
        }

        /// <summary>
        /// Rubyにあるようなやつ
        /// ex) 5.Times(i => Console.WriteLine(i));
        /// </summary>
        /// <param name="times"></param>
        /// <param name="action"></param>
        public static void Times(this int times, Action<int> action)
        {
            for (int i = 0; i < times; i++)
            {
                action(i);
            }
        }

        /// <summary>
        /// 引数aと引数bの値を入れ替えます
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="a"></param>
        /// <param name="b"></param>
        public static void Swap<T>(ref T a, ref T b)
        {
            T _a = a;
            a = b;
            b = _a;
        }

        /// <summary>
        /// Console.WriteLineの自動フラッシュをしないようにする
        /// </summary>
        public static void DontAutoFlush()
        {
            var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            Console.SetOut(sw);
        }

        public static void Flush()
        {
            Console.Out.Flush();
        }

        /// <summary>
        /// 先頭と末尾に要素を増やした新しい配列を返します。
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="array"></param>
        /// <returns></returns>
        public static T[] AddHeadAndTail<T>(T[] array)
        {
            int len = array.Length;
            T[] res = new T[len + 2];
            Array.Copy(array, 0, res, 1, len);
            return res;
        }

        /// <summary>
        /// 昇順ソート済みの配列を2分探索します。
        /// 要素が見つからなかった場合はnより大きい数値の中で最小の数値のインデックスを返す。
        /// </summary>
        /// <param name="array"></param>
        /// <param name="n"></param>
        /// <returns></returns>
        public static int BinarySearch(long[] array, long n)
        {
            int left = 0;
            int right = array.Length - 1;

            while (left <= right)
            {
                int mid = (right - left) / 2 + left;

                if (array[mid] == n)
                {
                    return mid;
                }
                else if (n < array[mid])
                {
                    right = mid - 1;
                }
                else
                {
                    left = mid + 1;
                }
            }

            return left;
        }

        public static int GCD(int a, int b)
        {
            return (b == 0) ? a : GCD(b, a % b);
        }

        public static int[] GetDivisor(int n)
        {
            double sqrt = Math.Sqrt(n);

            List<int> divisor = new List<int>();

            for (int i = 1; i <= sqrt; i++)
            {
                if (n % i == 0)
                {
                    divisor.Add(i);
                    int tmp = n / i;
                    if (tmp != i)
                    {
                        divisor.Add(tmp);
                    }
                }
            }

            return Sort(divisor.ToArray());
        }

        public static int[] LongToBinaryArray(long n)
        {
            List<int> lst = new List<int>();

            while (n > 0)
            {
                int amari = (int)(n % 2);
                lst.Add(amari);
                n /= 2;
            }

            return lst.ToArray();
        }

        public static long BinaryArrayToLong(int[] binaryArray)
        {
            long result = 0;
            long b = 1;
            int len = binaryArray.Length;

            for (int i = 0; i < len; i++)
            {
                result += binaryArray[i] * b;
                b *= 2;
            }

            return result;
        }

        public static void Deconstruct<T>(this T[] items, out T t0)
        {
            t0 = items.Length > 0 ? items[0] : default(T);
        }

        public static void Deconstruct<T>(this T[] items, out T t0, out T t1)
        {
            if (items.Length < 2)
                throw new ArgumentException();

            t0 = items[0];
            t1 = items[1];
        }

        public static void Deconstruct<T>(this T[] items, out T t0, out T t1, out T t2)
        {
            if (items.Length < 3)
                throw new ArgumentException();

            t0 = items[0];
            t1 = items[1];
            t2 = items[2];
        }

        public static void Deconstruct<T>(this T[] items, out T t0, out T t1, out T t2, out T t3)
        {
            if (items.Length < 4)
                throw new ArgumentException();

            t0 = items[0];
            t1 = items[1];
            t2 = items[2];
            t3 = items[3];
        }

        public static void ArrayInitialize<T>(T[,] array, T value)
        {
            int len0 = array.GetLength(0);
            int len1 = array.GetLength(1);

            for (int i = 0; i < len0; i++)
            {
                for (int j = 0; j < len1; j++)
                {
                    array[i, j] = value;
                }
            }
        }

        public static void ChMax(ref long a, long b)
        {
            if (a < b) a = b;
        }

        public static void ChMin(ref long a, long b)
        {
            if (a > b) a = b;
        }

        public readonly static int MOD = 1000000007;
        public static long ModAdd(long a, long b)
        {
            long res = a + b;
            if (res >= MOD)
                return res % MOD;
            return res;
        }
        public static long ModMul(long a, long b)
        {
            long res = a * b;
            if (res >= MOD)
                return res % MOD;
            return res;
        }

        public static void ModAddEqual(ref long a, long b)
        {
            a += b;
            if (a >= MOD)
                a %= MOD;
        }

        public static T MinBy<T>(this IEnumerable<T> source, Func<T, long> conv)
        {
            T min = source.First();
            long minValue = conv(min);
            foreach (T x in source.Skip(1))
            {
                long n = conv(x);
                if (n < minValue)
                {
                    min = x;
                }
            }

            return min;
        }

        public static T MaxBy<T>(this IEnumerable<T> source, Func<T, long> conv)
        {
            T min = source.First();
            long maxValue = conv(min);
            foreach (T x in source.Skip(1))
            {
                long n = conv(x);
                if (maxValue < n)
                {
                    min = x;
                }
            }

            return min;
        }

        public static void FillArray<T>(T[] array, T value)
        {
            int max = array.Length;
            for (int i = 0; i < max; i++)
            {
                array[i] = value;
            }
        }

        public static void FillArray<T>(T[,] array, T value)
        {
            int max0 = array.GetLength(0);
            int max1 = array.GetLength(1);

            for (int i = 0; i < max0; i++)
            {
                for (int j = 0; j < max1; j++)
                {
                    array[i, j] = value;
                }
            }
        }

        public static void FillArray<T>(T[,,] array, T value)
        {
            int max0 = array.GetLength(0);
            int max1 = array.GetLength(1);
            int max2 = array.GetLength(2);

            for (int i = 0; i < max0; i++)
            {
                for (int j = 0; j < max1; j++)
                {
                    for (int k = 0; k < max2; k++)
                    {
                        array[i, j, k] = value;
                    }
                }
            }
        }

    }

    /// <summary>
    /// HashSetにTupleを入れた時の、等値性判定方法の指定に使います。
    /// HashSetのコンストラクタに渡して使います。
    /// EqualsとGetHashCodeを提供します。
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public class EqualityComparer<T> : IEqualityComparer<T>
    {
        private Func<T, T, bool> _equals;
        private Func<T, int> _getHashCode;

        public EqualityComparer(Func<T, T, bool> equals, Func<T, int> getHashCode)
        {
            _equals = equals;
            _getHashCode = getHashCode;
        }

        public bool Equals(T x, T y)
        {
            return _equals(x, y);
        }

        public int GetHashCode(T obj)
        {
            return _getHashCode(obj);
        }
    }

    /// <summary>
    /// UnionFindTree
    /// </summary>
    public class UnionFindTree
    {
        public int[] Node;

        public UnionFindTree(int N)
        {
            Node = new int[N];
            for (int i = 0; i < N; i++)
            {
                Node[i] = -1;
            }
        }

        public bool IsSameGroup(int x, int y)
        {
            return GetRoot(x) == GetRoot(y);
        }

        public bool Merge(int x, int y)
        {
            int xr = GetRoot(x);
            int yr = GetRoot(y);

            if (xr == yr)
                return false;

            // xが、大きなグループを示すようにSwapする（値が小さい方が大きなグループ）
            if (Node[xr] > Node[yr])
                Swap(ref xr, ref yr);

            // グループの数を合算する
            Node[xr] += Node[yr];

            // 根を張り替える
            Node[yr] = xr;
            return true;
        }

        public int Size(int x)
        {
            return -Node[GetRoot(x)];
        }

        public int GetRoot(int n)
        {
            if (Node[n] < 0)
            {
                return n;
            }
            else
            {
                // 根を張りなおす。
                Node[n] = GetRoot(Node[n]);
                return Node[n];
            }
        }
    }

    /// <summary>
    /// PriorityQueue
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public class PriorityQueue<T>
    {
        private Comparison<T> _comparison = null;
        private int _type = 0;

        private T[] _heap;
        private int _sz = 0;

        private int _count = 0;

        /// <summary>
        /// Priority queue
        /// </summary>
        /// <param name="maxSize">max size</param>
        /// <param name="type">0: asc, 1:desc</param>
        public PriorityQueue(int maxSize, int type = 0)
        {
            _heap = new T[maxSize];
            _type = type;
        }

        public PriorityQueue(int maxSize, Comparison<T> comparison)
        {
            _heap = new T[maxSize];
            _comparison = comparison;
        }

        private int Compare(T x, T y)
        {
            return _comparison(x, y);
            //if (_comparison != null) return _comparison(x, y);
            //return _type == 0 ? x.CompareTo(y) : y.CompareTo(x);
        }

        public void Push(T x)
        {
            _count++;

            //node number
            var i = _sz++;

            while (i > 0)
            {
                //parent node number
                var p = (i - 1) / 2;

                if (Compare(_heap[p], x) <= 0) break;

                _heap[i] = _heap[p];
                i = p;
            }

            _heap[i] = x;
        }

        public T Pop()
        {
            _count--;

            T ret = _heap[0];
            T x = _heap[--_sz];

            int i = 0;
            while (i * 2 + 1 < _sz)
            {
                //children
                int a = i * 2 + 1;
                int b = i * 2 + 2;

                if (b < _sz && Compare(_heap[b], _heap[a]) < 0) a = b;

                if (Compare(_heap[a], x) >= 0) break;

                _heap[i] = _heap[a];
                i = a;
            }

            _heap[i] = x;

            return ret;
        }

        public int Count()
        {
            return _count;
        }

        public T Peek()
        {
            return _heap[0];
        }

        public bool Contains(T x)
        {
            for (int i = 0; i < _sz; i++) if (x.Equals(_heap[i])) return true;
            return false;
        }

        public void Clear()
        {
            while (this.Count() > 0) this.Pop();
        }

        public IEnumerator<T> GetEnumerator()
        {
            var ret = new List<T>();

            while (this.Count() > 0)
            {
                ret.Add(this.Pop());
            }

            foreach (var r in ret)
            {
                this.Push(r);
                yield return r;
            }
        }

        public T[] ToArray()
        {
            T[] array = new T[_sz];
            int i = 0;

            foreach (var r in this)
            {
                array[i++] = r;
            }

            return array;
        }
    }
}