CodeWars/4kyu/sudoku_solution_validator/solution.cs

using System.Collections.Generic;

public class Sudoku {
  private static readonly int FULL = (~(((~0) >> 9) << 9)) << 1;

  private static IEnumerable<(int, int)> Row(int row) {
    for (var col = 0; col < 9; col++) {
      yield return (row, col);
    }
  }

  private static IEnumerable<(int, int)> Column(int column) {
    for (var row = 0; row < 9; row++) {
      yield return (row, column);
    }
  }

  private static IEnumerable<(int, int)> Box(int k) {
    var row = 3 * (k / 3);
    var col = 3 * (k % 3);

    for (var i = 0; i < 3; i++) {
      for (var j = 0; j < 3; j++) {
        yield return (row + i, col + j);
      }
    }
  }

  private static bool Valid(int[][] board, IEnumerable<(int, int)> coordinates) {
    var encountered = 0;
    foreach (var (row, col) in coordinates) {
      if (board[row][col] == 0) {
        return false;
      } else if ((encountered & (1 << board[row][col])) != 0) {
        return false;
      }

      encountered |= 1 << board[row][col];
    }

    return encountered == FULL;
  }

  public static bool ValidateSolution(int[][] board) {
    for (var i = 0; i < 9; i++) {
      if (!Valid(board, Row(i)) || !Valid(board, Column(i)) || !Valid(board, Box(i))) {
        return false;
      }
    }

    return true;
  }
}