Sudoku leetcode 37 - practice 30 minutes, I used exactly 30 minutes to finish coding, testing, pass all test cases.

Sudoku_Leetcode37_Practice.cs

using System;

class Solution
{
    public static bool SudokuSolve(char[,] board) 
    {
      // your code goes here
      if(board == null || board.GetLength(0) == 0 || board.GetLength(1) == 0)
      {
        return false; 
      }
      
      return SudokuSolveHelper(board, 0, 0); 
    }

    // 9 * 9, row from 0 - 8
    private static bool SudokuSolveHelper(char[,] board, int row, int column)
    {
        if(row == 9)    
        {
          return true; 
        }
      
        // current visit is (row, column)
        var visit = board[row, column]; 
        var isDot = visit == '.'; 
      
       // move to next 
        var newRow    = column < 8? row : (row + 1); 
        var newColumn = column == 8? 0  : (column + 1); 
      
        if(!isDot)
        {                    
          return SudokuSolveHelper(board, newRow, newColumn); 
        }
        
        // assume that it is Dot
        var numbers = new char[]{'1','2','3','4','5', '6','7','8','9'}; 
        foreach(var number in numbers)
        {
          var checking = isAvailable(number, board, row, column); // by row, by column, by 3 * 3 matrix 
          if(!checking)
          {
            continue; 
          }
          
          board[row, column] = number; 
          if(SudokuSolveHelper(board, newRow, newColumn))
          {
            return true; 
          }
          
          board[row, column] = '.'; // backtracking 
        }
         
        return false; 
    }
  
    private static bool isAvailable(char selected, char[,] board, int row, int column)
    {
      // check by row
      for(int colIndex = 0; colIndex < 9; colIndex++)
      {
        var visit = board[row, colIndex]; 
        var found = selected == visit; 
        if(found)
        {
          return false; 
        }
      }
      
      // check by column
      for(int rowIndex = 0; rowIndex < 9; rowIndex++)
      {
        var visit = board[rowIndex, column]; 
        var found = selected == visit; 
        if(found)
        {
          return false; 
        }
      }
        
      // check by 3 * 3 matrix 
      // rowIndex 0, 1, 2 for 3 columns 0 - 0 - 2, 1 matchs from column 3 - 5, 2 matchs from 6 - 8
      // columnIndex 
      var matrixRow = row / 3 ; // 0, 1, 2 
      var matrixColumn = column / 3; 
      
      var startRow = matrixRow * 3; 
      var startColumn = matrixColumn * 3; 
      for(int rowIndex = startRow; rowIndex < startRow + 3; rowIndex++)
      {
        for(int colIndex =  startColumn; colIndex < startColumn + 3; colIndex++)
        {
          var visit = board[rowIndex, colIndex]; 
          var found = selected == visit; 
          if(found)
          {
            return false; 
          }
        }
      }      
      
      return true; 
    }
  
  
    static void Main(string[] args)
    {

    }
}

// DFS - recursive function
// (0,0) -> base case: row = 9, return true
// return false
// scan row by row, from left to right, 5 3 _ _ 
// available option 1 - 9, exclude 3, 5, 6 - row, exclude 8, by 3 * 3 matrix exclue 3, 5, 6, 8, 9
// 1, 2, 4, 7 -> 
// 1 -> recursive ok -> return true
// 2 -> 
// 4 -> 
// 7 -> 
// return false;