Valid Sudoku

Description

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(javascript)

/**
 * @param {character[][]} board
 * @return {boolean}
 */
const isValidSudoku = (board = []) => {
  const rows = []
  const columns = []
  const boxes = []

  for (let i = 0; i < board.length; i++) {
    for (let j = 0; j < board[i].length; j++) {
      const content = board[i][j]
      if (content !== '.') {
        if (!rows[i]) {
          rows[i] = {}
        }
        if (rows[i][content]) {
          return false
        }
        rows[i][content] = 1

        if (!columns[j]) {
          columns[j] = {}
        }
        if (columns[j][content]) {
          return false
        }
        columns[j][content] = 1
        const boxNum = Math.floor(i / 3) * 3 + Math.floor(j / 3)
        if (!boxes[boxNum]) {
          boxes[boxNum] = {}
        }
        if (boxes[boxNum][content]) {
          return false
        }
        boxes[boxNum][content] = 1
      }
    }
  }
  return true
}