Lecture 79 :-  String To Integer (atoi)

Converting a string to an integer, also known as the "atoi" (ASCII to Integer) problem, is a common coding interview question. The task is to convert a given string containing digits (and optional leading whitespace and sign) to its corresponding integer representation.

Here are the steps to implement the atoi function:

Remove leading whitespace from the string.

Check if the first non-whitespace character is a sign (+ or -). If it is, keep track of the sign of the integer.

Iterate through the remaining characters of the string until a non-digit character is encountered or until the end of the string.

Convert the consecutive digits into an integer, making sure to handle integer overflow.

Return the resulting integer, considering the sign we kept track of earlier.

Here's a Python function to implement atoi:

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def atoi(s):    # Remove leading whitespace    s = s.strip()    # Check for empty string or invalid input    if not s or (not s[0].isdigit() and s[0] not in ['+', '-']):        return 0    # Check for the sign    sign = 1    if s[0] == '-':        sign = -1        s = s[1:]    elif s[0] == '+':        s = s[1:]    # Convert the digits to an integer    result = 0    for char in s:        if char.isdigit():            result = result * 10 + int(char)        else:            break    # Apply the sign and handle integer overflow    result = max(min(sign * result, 2**31 - 1), -2**31)    return result # Example usage: input_str = "   -42" output_int = atoi(input_str) print(output_int)

Output:

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-42

In this example, the function atoi takes the input string s, processes it according to the steps mentioned above, and returns the resulting integer. The function handles leading whitespace, signs, and invalid inputs. It also takes care of integer overflow, ensuring the result falls within the range of a 32-bit signed integer (between -2^31 to 2^31 - 1).