using System;

​

static bool IsValidFast(int[,] board)

{

        for (var i = 0; i < 9; i++)

        {

            int rowMask = 0, colMask = 0, sqrMask = 0;

​

            for (var j = 0; j < 9; j++)

            {

                // Check row

                if (board[i, j] != 0)

                {

                    int rowBit = 1 << (board[i, j] - 1);

                    if ((rowMask & rowBit) != 0)

                        return false;

                    rowMask |= rowBit;

                }

​

                // Check column

                if (board[j, i] != 0)

                {

                    int colBit = 1 << (board[j, i] - 1);

                    if ((colMask & colBit) != 0)

                        return false;

                    colMask |= colBit;

                }

​

                // Check 3x3 square

                int r = 3 * (i / 3) + j / 3;

                int c = 3 * (i % 3) + j % 3;

                if (board[r, c] != 0)

                {

                    int sqrBit = 1 << (board[r, c] - 1);

                    if ((sqrMask & sqrBit) != 0)

                        return false;

                    sqrMask |= sqrBit;

                }

            }

        }

    return true;

}

​

var soduku = new [,]

        {

            {5,3,0,0,7,0,0,0,0},

            {6,0,0,1,9,5,0,0,0},

            {0,9,8,0,0,0,0,6,0},

            {8,0,0,0,6,0,0,0,3},

            {4,0,0,8,0,3,0,0,1},

            {7,0,0,0,2,0,0,0,6},

            {0,6,0,0,0,0,2,8,0},

            {0,0,0,4,1,9,0,0,5},

            {0,0,0,0,8,0,0,7,9}

        };

​

var result = IsValidFast(soduku);

Console.WriteLine(result);

​