Leetcode 36. Valid Sudoku

Valid Sudoku – LeetCode

Can you solve this real interview question? Valid Sudoku – Determine if a 9 x 9 Sudoku board is valid.

Problem Statement

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.

Examples

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

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

Use sets to keep track of the numbers seen so far for each row, column, and 3×3 sub-box.
Iterate through the board: Use nested loops to go through each cell in the board.
Check constraints: For each number in the board, check if it violates the Sudoku rules by checking the corresponding row, column, and sub-box sets.
Update sets: If a number does not violate the rules, add it to the appropriate sets.

Python:

def isValidSudoku(board):
    # Use a set to keep track of the seen numbers in rows, columns, and sub-boxes
    rows = [set() for _ in range(9)]
    cols = [set() for _ in range(9)]
    boxes = [set() for _ in range(9)]
    
    for r in range(9):
        for c in range(9):
            num = board[r][c]
            if num != '.':
                # Calculate the index for the 3x3 sub-box
                box_index = (r // 3) * 3 + (c // 3)
                
                if num in rows[r]:
                    return False
                rows[r].add(num)
                
                if num in cols[c]:
                    return False
                cols[c].add(num)
                
                if num in boxes[box_index]:
                    return False
                boxes[box_index].add(num)
    
    return True

Java:

class Solution {
    public boolean isValidSudoku(char[][] board) {
        // Use arrays of sets to keep track of the seen numbers in rows, columns, and sub-boxes
        Set<Character>[] rows = new Set[9];
        Set<Character>[] cols = new Set[9];
        Set<Character>[] boxes = new Set[9];
        
        for (int i = 0; i < 9; i++) {
            rows[i] = new HashSet<>();
            cols[i] = new HashSet<>();
            boxes[i] = new HashSet<>();
        }
        
        for (int r = 0; r < 9; r++) {
            for (int c = 0; c < 9; c++) {
                char num = board[r][c];
                if (num != '.') {
                    // Calculate the index for the 3x3 sub-box
                    int boxIndex = (r / 3) * 3 + (c / 3);
                    
                    // Check if the number is already in the respective row, column, or sub-box
                    if (rows[r].contains(num) || cols[c].contains(num) || boxes[boxIndex].contains(num)) {
                        return false;
                    }
                    
                    // Add the number to the respective row, column, and sub-box sets
                    rows[r].add(num);
                    cols[c].add(num);
                    boxes[boxIndex].add(num);
                }
            }
        }
        
        return true;
    }
}

Leetcode 36. Valid Sudoku

Leetcode 36. Valid Sudoku

Problem Statement

Examples

Constraints

Solution

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