Source: https://leetcode.com/problems/valid-sudoku/

Determine if a 9×9 Sudoku board is valid. Only the filled cells need to be validated **according to the following rules**:

- Each row must contain the digits
`1-9`

without repetition. - Each column must contain the digits
`1-9`

without repetition. - Each of the 9
`3x3`

sub-boxes of the grid must contain the digits`1-9`

without repetition.

A partially filled sudoku which is valid.

The Sudoku board could be partially filled, where empty cells are filled with the character `'.'`

.

**Example 1:**

Input:[ ["5","3",".",".","7",".",".",".","."], ["6",".",".","1","9","5",".",".","."], [".","9","8",".",".",".",".","6","."], ["8",".",".",".","6",".",".",".","3"], ["4",".",".","8",".","3",".",".","1"], ["7",".",".",".","2",".",".",".","6"], [".","6",".",".",".",".","2","8","."], [".",".",".","4","1","9",".",".","5"], [".",".",".",".","8",".",".","7","9"] ]Output:true

**Example 2:**

Input:[ ["8","3",".",".","7",".",".",".","."], ["6",".",".","1","9","5",".",".","."], [".","9","8",".",".",".",".","6","."], ["8",".",".",".","6",".",".",".","3"], ["4",".",".","8",".","3",".",".","1"], ["7",".",".",".","2",".",".",".","6"], [".","6",".",".",".",".","2","8","."], [".",".",".","4","1","9",".",".","5"], [".",".",".",".","8",".",".","7","9"] ]Output:falseExplanation:Same as Example 1, except with the5in the top left corner being modified to8. Since there are two 8's in the top left 3x3 sub-box, it is invalid.

**Note:**

- A Sudoku board (partially filled) could be valid but is not necessarily solvable.
- Only the filled cells need to be validated according to the mentioned rules.
- The given board contain only digits
`1-9`

and the character`'.'`

. - The given board size is always
`9x9`

.

We solve this problem according to the three rules. First, we check every row in the datasets with repetition. Then we check every column in the datasets with repetition. Finally, we check every `3x3`

sub-box.

We can use hashmap to check repetition, but there are only 9 numbers we care about. So I decided to use 9 elements array to count numbers.

Any array contains a number bigger than 1, then the sudoku is invalid.

Then we check every row:

Every Column:

Every `3x3`

sub-box:

Then we combine these three rules’ results:

Github：https://github.com/tinyfool/leetcode/tree/master/src/p0036

To see more hashtable related problems, see

Problems and Solutions of Hashmap, HashSet.