36. Valid Sudoku

Determine if a 9 x 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 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.

ex1

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Example 1:

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

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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 ‘.’.

Solution

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class Solution {
    public boolean isValidSudoku(char[][] board) {
        for (int i = 0; i < 9; i++) {
            if (!isValidRow(i, board)) {
                return false;
            }
            if (!isValidCol(i, board)) {
                return false;
            }
        }
        
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                if (i % 3 == 0 && j % 3 == 0) {
                    if (!isValidBox(i, j, board)) {
                        return false;
                    }
                }
            }
        }
        return true;
    }
    
    boolean isValidRow(int i, char[][] board) {
        Set<Integer> set = new HashSet<>();
        for (int j = 0; j < 9; j++) {
            if (board[i][j] == '.') {
                continue;
            }
            int val = board[i][j] - '0';
            if (!set.add(val)) {
                return false;
            }
        }
        return true;
    }
    
    boolean isValidCol(int j, char[][] board) {
        Set<Integer> set = new HashSet<>();
        for (int i = 0; i < 9; i++) {
            if (board[i][j] == '.') {
                continue;
            }
            int val = board[i][j] - '0';
            if (!set.add(val)) {
                return false;
            }
        }
        return true;
    }
    
    boolean isValidBox(int i, int j, char[][] board) {
        Set<Integer> set = new HashSet<>();
        for (int x = i; x < i + 3; x++) {
            for (int y = j; y < j + 3; y++) {
                if (board[x][y] == '.') {
                    continue;
                }
                int val = board[x][y] - '0';
                if (!set.add(val)) {
                    return false;
                }
            }
        }
        return true;
    }
}

Solution#

Solution