[LeetCode] 37. Sudoku Solver

Problem

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

A sudoku solution must satisfy all of the following rules:

Each of the digits 1-9 must occur exactly once in each row.
Each of the digits 1-9 must occur exactly once in each column.
Each of the the digits 1-9 must occur exactly once in each of the 9 3x3 sub-boxes of the grid.
Empty cells are indicated by the character '.'.

A sudoku puzzle...

...and its solution numbers marked in red.

Note:

The given board contain only digits 1-9 and the character '.'.
You may assume that the given Sudoku puzzle will have a single unique solution.
The given board size is always 9x9.

Solution

class Solution {
    public void solveSudoku(char[][] board) {
        if (board == null || board.length == 0) return;
        solve(board);
    }
    
    private 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 ch = '1'; ch <= '9'; ch++) {
                        if (isValid(board, i, j, ch)) {
                            board[i][j] = ch;
                            if (solve(board)) return true;
                            else board[i][j] = '.';
                        }
                    }
                    return false;
                }
                
            }
        }
        return true;
    }
    
    private boolean isValid(char[][] board, int row, int col, char ch) {
        for (int i = 0; i < 9; i++) {
            if (board[i][col] == ch) return false;
            if (board[row][i] == ch) return false;
            if (board[row/3*3+i/3][col/3*3+i%3] == ch) return false;
        }
        return true;
    }
}