function isValid(board, row,col,k){

  for(let i=0;i<9;i++){
    if(board[row][i]===k || board[i][col]===k){
      return false
    }
  }
  const x = Math.floor(row/3) *3
  const y = Math.floor(col/3) *3
  for(let i=0;i<3;i++){
    for(let j=0;j<3;j++){
      if(board[x+i][y+j]===k){
        return false
      }
    }
  }
  return true

}
@param {character[][]} board
@return {void} Do not return anything, modify board in-place instead.