36. Valid Sudoku (Medium)

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

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.

Solutions

class Solution {
    public boolean isValidSudoku(char[][] board) {
        int row = board.length;
        int col = board[0].length;

        // check rows
        for (int i = 0; i < row; i++) {
            Set<Character> set = new HashSet<>();
            for (int j = 0; j < col; j++) {
                if (board[i][j] != '.' && !set.add(board[i][j])) {
                    return false;
                }
            }
        }

        // check columns
        for (int i = 0; i < col; i++) {
            Set<Character> set = new HashSet<>();
            for (int j = 0; j < row; j++) {
                if (board[j][i] != '.' && !set.add(board[j][i])) {
                    return false;
                }
            }
        }

        // check sub squares
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 3; j++) {
                Set<Character> set = new HashSet<>();
                for (int x = i * 3; x < i * 3 + 3; x++) {
                    for (int y = j * 3; y < j * 3 + 3; y++) {
                        if (board[x][y] != '.' && !set.add(board[x][y])) {
                            return false;
                        }
                    }
                }
            }
        }

        return true;
    }
}

Incorrect Solutions

References

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