37. Sudoku Solver

SudokuSolver.java

public class Solution {
    // 37. Sudoku Solver
    // https://leetcode.com/problems/sudoku-solver/
    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
                        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;
    }
    private boolean isValid(char[][] board, int row, int col, char c){
        int regionRow = 3 * (row / 3);  //region start row
        int regionCol = 3 * (col / 3);    //region start col
        // System.out.println("row: "+ row + " col: " + col + " regionRow: " + regionRow+ " regionCol: " + regionCol);
        for (int i = 0; i < 9; i++) {
            
            if (board[i][col] == c) return false; //check row
            if (board[row][i] == c) return false; //check column
            // System.out.println("row: "+ (regionRow + i / 3) + " col: " + (regionCol + i % 3));
            if (board[regionRow + i / 3][regionCol + i % 3] == c) return false; //check 3*3 block
        }
        return true;
    }
}