Leetcode-Sudoku Solver

 1 public class Solution {
 2     public void solveSudoku(char[][] board) {
 3         if (board.length!=9) return;
 4         if (board[0].length!=9) return;
 5 
 6         //Construct the available set for each point.
 7         List<Set<Integer>> rowAva = new ArrayList<Set<Integer>>();
 8         List<Set<Integer>> colAva = new ArrayList<Set<Integer>>();
 9         List<Set<Integer>> areaAva = new ArrayList<Set<Integer>>();
10         Set<Integer> base = new HashSet<Integer>();
11         for (int i=1;i<=9;i++) base.add(i);
12         for (int i=0;i<9;i++){
13             Set<Integer> temp = new HashSet<Integer>();
14             temp.addAll(base);
15             rowAva.add(temp);
16             temp = new HashSet<Integer>();
17             temp.addAll(base);
18             colAva.add(temp);
19             temp = new HashSet<Integer>();
20             temp.addAll(base);
21             areaAva.add(temp);
22         }
23         //Preprocessing: take out each exsiting number from the ava set of the row and the col.
24         for (int i=0;i<9;i++)
25             for (int j=0;j<9;j++)
26                 if (board[i][j]!=‘.‘){
27                     int val = board[i][j]-‘0‘;
28                     rowAva.get(i).remove(val);
29                     colAva.get(j).remove(val);
30                     int ind = (i/3)*3+(j/3);
31                     areaAva.get(ind).remove(val);
32                 }
33     
34         //Recursively find out the solution.
35         solveSudokuRecur(board,0,0,rowAva,colAva,areaAva);
36     }
37 
38     public boolean solveSudokuRecur(char[][] board, int x, int y, List<Set<Integer>> rowAva, List<Set<Integer>> colAva, List<Set<Integer>> areaAva){
39         if (x==9) return true;
40 
41         char cur = board[x][y];
42         int nextX = (x*9+y+1)/9;
43         int nextY = (x*9+y+1)%9;
44         //it is filled.
45         if (cur!=‘.‘)  return solveSudokuRecur(board,nextX,nextY,rowAva,colAva,areaAva);
46         //if it is not filled, then try to fill every possible value.
47         int ind = (x/3)*3+(y/3);
48         for (int i=1;i<=9;i++)
49             if (rowAva.get(x).contains(i) && colAva.get(y).contains(i) && areaAva.get(ind).contains(i)){
50                 board[x][y] = (char)(i+‘0‘);
51                 //remove i from ava sets.
52                 rowAva.get(x).remove(i);
53                 colAva.get(y).remove(i);
54                 areaAva.get(ind).remove(i);                
55                 boolean valid = solveSudokuRecur(board,nextX,nextY,rowAva,colAva,areaAva);
56                 if (valid) return true;
57                 //backtracking.
58                 areaAva.get(ind).add(i);
59                 colAva.get(y).add(i);
60                 rowAva.get(x).add(i);
61                 board[x][y]=‘.‘;
62             }
63         return false;
64     }
65             
66             
67 }