# LeetCode 8: String to Integer (atoi)

I'm posting a solution for LeetCode's "String to Integer (atoi)". If you'd like to review, please do. Thank you!

### Problem

Implement atoi which converts a string to an integer. The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value. The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function. If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed. If no valid conversion could be performed, a zero value is returned.

Note:

Only the space character ' ' is considered a whitespace character. Assume we are dealing with an environment that could only store integers within the 32-bit signed integer range: [−231, 231 − 1]. If the numerical value is out of the range of representable values, 231 − 1 or −231 is returned.

Example 1:

Input: str = "42"
Output: 42


Example 2:

Input: str = "   -42"
Output: -42
Explanation: The first non-whitespace character is '-', which is the minus sign. Then take as many numerical digits as possible, which gets 42.


Example 3:

Input: str = "4193 with words"
Output: 4193
Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.


Example 4:

Input: str = "words and 987"
Output: 0
Explanation: The first non-whitespace character is 'w', which is not a numerical digit or a +/- sign. Therefore no valid conversion could be performed.


Example 5:

Input: str = "-91283472332"
Output: -2147483648
Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer. Thefore INT_MIN (−231) is returned.


### Code

from typing import List
import collections
import itertools
import functools
import math
import string
import random
import bisect
import re
import operator
import heapq
import queue

from queue import PriorityQueue
from itertools import combinations, permutations
from functools import lru_cache
from collections import defaultdict
from collections import OrderedDict
from collections import deque
from collections import Counter

class Solution:
def myAtoi(self, s):
s = re.findall(r'^\s*[+-]?\d+', s)
try:
MAX, MIN = 2147483647, -2147483648
res = int(''.join(s))
if res > MAX:
return MAX
if res < MIN:
return MIN
return res
except:
return 0

if __name__ == "__main__":
print(Solution().myAtoi("   -42"))


### References:

Nice solution, it's compact and simple to understand. There is little to improve, few suggestions:

• Imports: there are a lot of imports, maybe leftovers of previous attempts.
• Try Except block: should be around the code that can cause the exception.

Applying the suggestions:

import re

class Solution:
def myAtoi(self, s: str) -> int:
MAX, MIN = 2147483647, -2147483648
s = re.findall(r'^\s*[+-]?\d+', s)
try:
res = int(''.join(s))
except:
return 0
if res > MAX:
return MAX
if res < MIN:
return MIN
return res


## Performance

Runtime: 36 ms, faster than 51.56% of Python3 online submissions
Memory Usage: 14.1 MB, less than 32.27% of Python3 online submissions


Regex makes the code compact but is not the fastest approach. Faster solutions iterate through the string character by character.

## Magic numbers

2147483647, -2147483648


are actually

1<<31 - 1, -(1<<31)


which better conveys your intent: the limits of a signed 32-bit integer.

Consider putting a

DIGIT_PATTERN = re.compile(r'^\s*[+-]?\d+')


in global scope, so that multiple calls to myAtoi are faster.

## Never bare except

You should likely instead except ValueError, which is more narrow and well-defined.