LeetCode: Sudoku Solver

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.

A sudoku puzzle...

...and its solution numbers marked in red.

import java.util.Set;
import java.util.HashSet;

public class Solution {
    
    public boolean isValid(char[][] board, int a, int b){
        boolean[] flag = new boolean[9];
        
        
        Set<Character> contained = new HashSet<Character>();
        for(int j=0;j<9;j++){
            if(contained.contains(board[a][j])) return false;
            if(board[a][j]>'0' && board[a][j]<='9')
                contained.add(board[a][j]);
        }
            
        
    
        contained = new HashSet<Character>();
        for(int j=0;j<9;j++){
            if(contained.contains(board[j][b])) return false;
            if(board[j][b]>'0' && board[j][b]<='9')
                contained.add(board[j][b]);
        }
        
    
        contained = new HashSet<Character>();
        for(int m=0;m<3;m++){
            for(int n=0;n<3;n++){
                int x=a/3*3+m, y=b/3*3+n;
                if(contained.contains(board[x][y])) return false;
                    if(board[x][y]>'0' && board[x][y]<='9')
                        contained.add(board[x][y]);
            }  
        }
    
        return true;
    }
    
    public boolean solveSudoku(char[][] board) {
        // Start typing your Java solution below
        // DO NOT write main() function
        for(int i=0;i<9;i++){
            for(int j=0;j<9;j++){
                if(board[i][j]=='.'){
                    for(int k=0;k<9;k++){
                        board[i][j]=(char)('1'+k);
                        if(isValid(board,i,j) && solveSudoku(board)) return true;
                        board[i][j]='.';
                    }
                    return false;
                }
            }
        }
        return true;   
    }
}