第七篇 Valid Sudoku

Uber 高频题，细节实现，没什么太多可说的

注意检查3*3 square可能会比较难想

 1 public boolean isValidSudoku(char[][] board) {
 2         if (board == null || board.length == 0) {
 3             return false;
 4         }
 5         //O(MN) time
 6         //O(N) space
 7         int n = board.length;
 8         int m = board[0].length;
 9         HashSet<Character> set = new HashSet();
10         //Check rows
11         for (int i = 0; i < n; i++) {
12             for (int j = 0; j < m; j++) {
13                 if (board[i][j] != ‘.‘) {
14                     if (set.contains(board[i][j])) {
15                         //System.out.println("Row false " + board[i][j]);
16                         return false;
17                     }
18                     set.add(board[i][j]);
19                 }
20             }
21             set.clear();
22         }
23         set.clear();
24         //Check columns
25         for (int i = 0; i < m; i++) {
26             for (int j = 0; j < n; j++) {
27                 if (board[j][i] != ‘.‘) {
28                     if (set.contains(board[j][i])) {
29                         //System.out.println("Column false " + board[j][i]);
30                         return false;
31                     }
32                     set.add(board[j][i]);
33                 }
34             }
35             set.clear();
36         }
37         set.clear();
38         //Check squares
39         for (int i = 0; i < n - 2; i += 3) {
40             for (int j = 0; j < m - 2; j += 3) {
41                 for (int k = 0; k < 9; k ++) {
42                     if (board[i + k / 3][j + k % 3] != ‘.‘) {
43                         if (set.contains(board[i + k / 3][j + k % 3])) {
44                             return false;
45                         }
46                         set.add(board[i + k / 3][j + k % 3]);
47                     }
48                 }
49                 set.clear();
50             }
51         }
52         return true;
53     }