解数独

编写一个程序，通过填充空格来解决数独问题。
 
数独的解法需 遵循如下规则：
 
数字 1-9 在每一行只能出现一次。
数字 1-9 在每一列只能出现一次。
数字 1-9 在每一个以粗实线分隔的 3x3 宫内只能出现一次。（请参考示例图）
数独部分空格内已填入了数字，空白格用 '.' 表示。
 
示例 ：
输入：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"]]
输出：[["5","3","4","6","7","8","9","1","2"],["6","7","2","1","9","5","3","4","8"],["1","9","8","3","4","2","5","6","7"],["8","5","9","7","6","1","4","2","3"],["4","2","6","8","5","3","7","9","1"],["7","1","3","9","2","4","8","5","6"],["9","6","1","5","3","7","2","8","4"],["2","8","7","4","1","9","6","3","5"],["3","4","5","2","8","6","1","7","9"]]
 

/**
 * @param {character[][]} board
 * @return {void} Do not return anything, modify board in-place instead.
 */
var solveSudoku = function(board) {
  const nextCell = getNextCell(board);
  if (!nextCell.length) {
    return;
  }
  solve(board, ...nextCell);
  console.log(board);
  return;
};
 
function solve(board, x, y) {
  if (x === undefined || y === undefined) {
    return true;
  }
  const availableNumbers = getAvailableNumbers(board, x, y);
  const nextCell = getNextCell(board, x, y+1);
  if (availableNumbers.size === 0) {
    return false
  }
  for (const number of availableNumbers) {
    board[x][y] = number;
    if (solve(board, ...nextCell)) {
      return true
    }
  }
  board[x][y] = '.';
  return false;
}
 
function getNextCell(board, x = 0, y = 0) {
  for (let i = x; i < 9; i++) {
    for (let j = y; j < 9; j++) {
      if (board[i][j] === '.') {
        return [i, j];
      }
    }
    y = 0;
  }
  return [];
}
 
function getAvailableNumbers(board, x, y) {
  const availableNumbers = new Set(["1", "2", "3", "4", "5", "6", "7", "8", "9"]);
  const row = board[x];
  const col = board.map(row => row[y]);
  const square = [];
  const squareX = Math.floor(x / 3);
  const squareY = Math.floor(y / 3);
  for (let i = 0; i < 3; i++) {
    for (let j = 0; j < 3; j++) {
      square.push(board[squareX * 3 + i][squareY * 3 + j]);
    }
  }
  const numbers = [...row, ...col, ...square];
  for (const number of numbers) {
    if (number !== '.')
    availableNumbers.delete(number);
  }
  return availableNumbers;
}