Check if Valid Sudoku Blocks in Java

The challenge of solving valid Sudoku blocks

Determine if a 9×9 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 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.

The solution written in Java

class Solution { // Store our blocks Set blockSet = new HashSet<>(); // Perform the check public boolean isValidSudoku(char[][] board) { // Row and Column variables to check against Set rowset = new HashSet<>(); Set colset = new HashSet<>(); // Loop through the column for(int i = 0; i<9; i++){ // Loop through the row for(int j = 0; j<9; j++){ // False if exists in column if(colset.contains(board[i][j])) return false; // False if exists in row if(rowset.contains(board[j][i])) return false; // Add to column and row if(board[i][j] != '.') colset.add(board[i][j]); if(board[j][i] != '.') rowset.add(board[j][i]); } // Clear this run at the end rowset.clear(); colset.clear(); } // While we loop the board int i = 0, j = 0; while(i < 9 && j < 9) { while(i < 9) { // False if not valid if(!isValid(board, i, j)) return false; // Increment one section i += 3; // Clear the board blockSet.clear(); } // Next column/row i = 0; j += 3; } // Is valid! return true; } // Check if a section is valid private boolean isValid(char[][] board, int i, int j) { // Loop the column for(int x = i; x < i+3; x++) { // Loop the row for(int y = j; y < j+3; y++) { // False if already in the set if(blockSet.contains(board[x][y])) return false; // If still in a section, add it if(board[x][y] != '.') blockSet.add(board[x][y]); } } // Is valid! return true; } }
Code language: Java (java)
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