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Java-based LeetCode algorithm problem solutions, regularly updated

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36\. Valid Sudoku

Medium

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

1.  Each row must contain the digits `1-9` without repetition.
2.  Each column must contain the digits `1-9` without repetition.
3.  Each of the nine `3 x 3` sub-boxes of the grid must contain the digits `1-9` without repetition.

**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.

**Example 1:**

![](https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Sudoku-by-L2G-20050714.svg/250px-Sudoku-by-L2G-20050714.svg.png)

**Input:**

    board =
    [["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:**

    board =
    [["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:** false

**Explanation:** Same as Example 1, except with the **5** in the top left corner being modified to **8**. Since there are two 8's in the top left 3x3 sub-box, it is invalid. 

**Constraints:**

*   `board.length == 9`
*   `board[i].length == 9`
*   `board[i][j]` is a digit `1-9` or `'.'`.