LeetCode 36. Valid Sudoku

36. Valid Sudoku

Difficulty: Medium

Determine if a 9x9 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 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: 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.

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.

Solution

Language: Java

class Solution {
    public boolean isValidSudoku(char[][] board) {
        if (board == null || board.length != 9 || board[0].length != 9) {
            return false;
        }
        Set<Character> set = new HashSet<>();
        for (int i = 0; i < 9; i++) {
            set.clear();
            for (int j = 0; j < 9; j++) {
                char c = board[i][j];
                if (c != '.') {
                    if (set.contains(c)) {
                        return false;
                    }
                    set.add(c);
                }
            }
        }
        for (int i = 0; i < 9; i++) {
            set.clear();
            for (int j = 0; j < 9; j++) {
                char c = board[j][i];
                if (c != '.') {
                    if (set.contains(c)) {
                        return false;
                    }
                    set.add(c);
                }
            }
        }
        for (int i = 0; i < 9; i += 3) {
            for (int j = 0; j < 9; j += 3) {
                set.clear();
                for (int k = i; k < i + 3; k++) {
                    for (int m = j; m < j + 3; m++) {
                        char c = board[k][m];
                        if (c != '.') {
                            if (set.contains(c)) {
                                return false;
                            }
                            set.add(c);
                        }
                    }
                }
            }
        }
        return true;
    }
}