Sudoku Solver

Source

• leetcode: Sudoku Solver

Write a program to solve a Sudoku puzzle by filling the empty cells.

Empty cells are indicated by the character '.'.

You may assume that there will be only one unique solution.

Java

public class Solution {
    public void solveSudoku(char[][] board) {
        if(board == null || board.length == 0)
            return;
        solve(board);        
    }

    public boolean solve(char[][] board){
        for(int i = 0; i < board.length; i++){
            for(int j = 0; j < board[0].length; j++){
                if(board[i][j] == '.'){
                    for(char c = '1'; c <= '9'; c++){//trial. Try 1 through 9 for each cell
                        if(isValid(board, i, j, c)){
                            board[i][j] = c; //Put c for this cell

                            if(solve(board))
                                return true; //If it's the solution return true
                            else
                                board[i][j] = '.'; //Otherwise go back
                        }
                    }
                    return false;
                }
            }
        }
        return true;
    }

    public boolean isValid(char[][] board, int i, int j, char c){
        //Check colum
        for(int row = 0; row < 9; row++)
            if(board[row][j] == c)
                return false;

        //Check row
        for(int col = 0; col < 9; col++)
            if(board[i][col] == c)
                return false;

        //Check 3 x 3 block
        for(int row = 3*(i/3); row<3*(i/3)+3; row++)
            for(int col = 3*(j/3); col<3*(j/3)+3; col++)
                if(board[row][col] == c)
                    return false;
        return true;
    }
}