[LeetCode] Sudoku Solver [Backtracking]

Problem

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.

Solution

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

public class Solution {
    public void solveSudoku(char[][] board) {
        solve(board, 0, 0);
    }
    private boolean solve(char[][] board, int row, int col) {
        if (row == 9) return true;
        int nextRow = col == 8 ? row+1 : row;
        int nextCol = col == 8 ? 0 : col+1;
        if (board[row][col] != '.') return solve(board, nextRow, nextCol);
        for (int i = 1; i <= 9; i++) {
            board[row][col] = (char) ('0'+i);
            if (isValid(board, row, col) && solve(board, nextRow, nextCol)) return true;
        }
        board[row][col] = '.';
        return false;
    }
    public boolean isValid(char[][] board, int row, int col) {
        char cur = board[row][col];
        for (int i = 0; i < 9; i++) {
            if (i != row && cur == board[i][col]) return false;
        }
        for (int i = 0; i < 9; i++) {
            if (i != col && cur == board[row][i]) return false;
        }
        int n = board.length;
        int rowbox = row/3, colbox = col/3;
        for (int i = rowbox*3; i < rowbox*3+3; i++) {
            for (int j = colbox*3; j < colbox*3+3; j++) {
                if (board[i][j] == cur && (i != row || j != col)) return false;
            }
        }
        return true;
    }
}