Sudoku Solver

Leetcode 37.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
class Solution {
    public void solveSudoku(char[][] board) {
      backtricking(board);
    }
    private boolean backtricking(char[][] board){
        //for loop: outside for loop rows, inside for loop columns 
        for(int i = 0; i < 9;i++){ //loop rows
            for(int j = 0; j < 9;j++){ //loop columns 
               //Skip original numbers
               if(board[i][j] != '.') 
                 continue; 
                
                //Start looping 1-9 to see which one is appropriate here
                for(char k = '1'; k <= '9';k++){
                    if(isValidSudoku(i,j,k,board)){
                        board[i][j] = k;
                        //find the appropriate one, return true 
                        if(backtricking(board))
                           return true;
                        //undo previous operations 
                        board[i][j] = '.';
                    }
                }
                //after looping 1 - 9, the appropriate one cannot be found, return false 
                return false;
            }
        }
        //if there is no false return, which means that appropriate positions have been found on the board
        return true; 
    }
    //There are three dimensions to determine whether a chessboard is valid:
    // No duplicates in each row
    // No duplicates in each column 
    // No duplicates in the sub boxes of the grid 
    public boolean isValidSudoku(int row, int col, char val, char[][] board){
        //No duplicates in each row 
        for(int i = 0; i < 9;i++){
            if(board[row][i] == val)
              return false;
        }
        //No duplicates in each column 
        for(int i = 0;i < 9;i++){
            if(board[i][col] == val)
             return false;
        }
        //No duplicates in sub boxes 
        int startRow = (row / 3) * 3;
        int startColumn = (col / 3) * 3; 
        for(int i = startRow; i < startRow + 3;i++){
            for(int j = startColumn; j < startColumn + 3;j++){
                if(board[i][j] == val)
                  return false;
            }
        }
      //if there is no false return 
      return true;  
    }
}