判定字符串是否是整数

def strToint(strs):
  def isNumber(c):
    return c>="0" and c<="9"
  if strs==None:
    return strs
  i=0
  res=0
  flag=False
  if list(strs)[i]=="-":
    flag=True
    i+=1
  if list(strs)[i]=="+":
    i+=1
  while i<len(strs):
    if isNumber(list(strs)[i]):
      res= res*10+ord(list(strs)[i])-ord('0')
    else:
      flag=False
      print("不是数字")
      return -1
    i+=1
  return -res if flag else res