String to Integer (atoi)

Implement the myAtoi function to convert a string to a 32-bit signed integer. The conversion involves ignoring leading whitespace, determining the sign, reading digits until a non-digit or end of string, and finally clamping the result to the 32-bit signed integer range.

Approach

The author's notes outline a step-by-step process for implementing the myAtoi function. First, leading whitespace is skipped. Then, the sign (+ or -) is determined if present and the string ans is not already populated. Next, any non-digit characters encountered will cause the process to break. If digits are found, they are appended to the ans string. After iterating through the input string, if ans is empty or only contains a sign, 0 is returned. Otherwise, ans is converted to an integer and then checked against the 32-bit signed integer range, returning the clamped value if it falls outside.

Solution

class Solution:
    def myAtoi(self, s: str) -> int:
        ans = ""
        for char in s:
            if char == " ":
                if ans == "":
                    continue
                else:
                    break
            elif char == "-" or char == "+":
                if ans != "":
                    break
                else:
                    ans += char
            elif not char.isdigit():
                break
            else:
                ans += char

        if ans == "" or ans == "+" or ans == "-":
            return 0

        res = int(ans)
        if res > (2**31)-1 :
            return (2**31)-1
        elif res < (-2**31):
            return (-2**31)
        else:
            return res

Complexity

• Time: O(N) — We iterate through the input string s once.
• Space: O(N) — In the worst case, the ans string can store all characters of s if they are all valid for conversion.