```
class Solution:
# @return an integer
def atoi(self, str):
str = str.strip()
if len(str) == 0:
return 0
tmp = "0"
result = 0
i = 0
if str[0] in "+-":
tmp = str[0]
i = 1
MAX_INT = 2147483647
MIN_INT = -2147483648
for i in xrange(i, len(str)):
if str[i].isdigit():
tmp += str[i]
else:
break
if len(tmp) > 1:
result = int(tmp)
if result > MAX_INT > 0:
return MAX_INT
elif result < MIN_INT < 0:
return MIN_INT
else:
return result
```

At first, I remove all whitspaces by using strip() function. Then, we should know only one operator is allowed, if there are two operator, for example, "+-2", the input is invalid for atoi() function, 0 should be returned. And we should put as many as possible numerical value into the 'tmp' string. If the length of 'tmp' is 1, it means the 'tmp' is '0' or '+' or '-', and we should return 0 in the function. If the length of 'tmp' is greater than 1, it means the 'tmp' is '0...(numbers)' or '+...(numbers)' or '-...(numbers)', then we can use integer force conversion to make 'tmp' to an integer. At last, If the result is out of the range of representable values, MAX_INT (2147483647) or MIN_INT (-2147483648) is returned.