String to Integer (atoi)

String Parsing

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.

For example:

Input: str = "42"
Output: 42

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.

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

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.

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.

Thought Process

• Since we are dealing with whitespaces, we have to continue iterating until we find the first non-whitespace.

• To check if the current character is a valid number we can convert, we can have a set of valid numbers to check agaisnt.

• To handle the case for overflow:

Solution

class Solution:
    def myAtoi(self, s: str) -> int:
        i = 0
        nums = set("0123456789")
        res = 0
        neg = False
        MIN_INT = -2 ** 31 #-2147483648
        MAX_INT = 2 ** 31-1 #2147483647
        
        while i < len(s) and s[i] == " ":
            i+=1
        if i < len(s) and s[i] == '-':
            neg = True
            i+=1
        elif i < len(s) and s[i] == '+':
            i+=1
            
        while i < len(s) and s[i] in nums: 
            c = ord(s[i]) - ord('0')
            if res > MAX_INT // 10 or (res == MAX_INT // 10 and c > 7):
                return MIN_INT if neg else MAX_INT
            res = res * 10 + c  
            i+=1
        
        if neg:
            res*=-1
        
        return res
            

Time Complexity

• Time: O(n)

• Space: O(1)