String to Integer (atoi)

Medium

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Topics:

Strings

Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer.

The algorithm for myAtoi(string s) is as follows:

Whitespace: Ignore any leading whitespace (" ").
Signedness: Determine the sign by checking if the next character is '-' or '+', assuming positivity if neither present.
Conversion: Read the integer by skipping leading zeros until a non-digit character is encountered or the end of the string is reached. If no digits were read, then the result is 0.
Rounding: If the integer is out of the 32-bit signed integer range [-2³¹, 2³¹ - 1], then round the integer to remain in the range. Specifically, integers less than -2³¹ should be rounded to -2³¹, and integers greater than 2³¹ - 1 should be rounded to 2³¹ - 1.

Return the integer as the final result.

Example 1:

Input: s = "42"

Output: 42

Explanation:

The underlined characters are what is read in and the caret is the current reader position.
Step 1: "42" (no characters read because there is no leading whitespace)
         ^
Step 2: "42" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "42" ("42" is read in)
           ^

Example 2:

Input: s = " -042"

Output: -42

Explanation:

Step 1: "   -042" (leading whitespace is read and ignored)
            ^
Step 2: "   -042" ('-' is read, so the result should be negative)
             ^
Step 3: "   -042" ("042" is read in, leading zeros ignored in the result)
               ^

Example 3:

Input: s = "1337c0d3"

Output: 1337

Explanation:

Step 1: "1337c0d3" (no characters read because there is no leading whitespace)
         ^
Step 2: "1337c0d3" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "1337c0d3" ("1337" is read in; reading stops because the next character is a non-digit)
             ^

Example 4:

Input: s = "0-1"

Output: 0

Explanation:

Step 1: "0-1" (no characters read because there is no leading whitespace)
         ^
Step 2: "0-1" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "0-1" ("0" is read in; reading stops because the next character is a non-digit)
          ^

Example 5:

Input: s = "words and 987"

Output: 0

Explanation:

Reading stops at the first non-digit character 'w'.

Constraints:

0 <= s.length <= 200
s consists of English letters (lower-case and upper-case), digits (0-9), ' ', '+', '-', and '.'.

String to Integer (atoi)

Medium

Databricks

1 view

Topics:

Strings

Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer.

The algorithm for myAtoi(string s) is as follows:

Whitespace: Ignore any leading whitespace (" ").
Signedness: Determine the sign by checking if the next character is '-' or '+', assuming positivity if neither present.
Conversion: Read the integer by skipping leading zeros until a non-digit character is encountered or the end of the string is reached. If no digits were read, then the result is 0.
Rounding: If the integer is out of the 32-bit signed integer range [-2³¹, 2³¹ - 1], then round the integer to remain in the range. Specifically, integers less than -2³¹ should be rounded to -2³¹, and integers greater than 2³¹ - 1 should be rounded to 2³¹ - 1.

Return the integer as the final result.

Example 1:

Input: s = "42"

Output: 42

Explanation:

The underlined characters are what is read in and the caret is the current reader position.
Step 1: "42" (no characters read because there is no leading whitespace)
         ^
Step 2: "42" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "42" ("42" is read in)
           ^

Example 2:

Input: s = " -042"

Output: -42

Explanation:

Step 1: "   -042" (leading whitespace is read and ignored)
            ^
Step 2: "   -042" ('-' is read, so the result should be negative)
             ^
Step 3: "   -042" ("042" is read in, leading zeros ignored in the result)
               ^

Example 3:

Input: s = "1337c0d3"

Output: 1337

Explanation:

Step 1: "1337c0d3" (no characters read because there is no leading whitespace)
         ^
Step 2: "1337c0d3" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "1337c0d3" ("1337" is read in; reading stops because the next character is a non-digit)
             ^

Example 4:

Input: s = "0-1"

Output: 0

Explanation:

Step 1: "0-1" (no characters read because there is no leading whitespace)
         ^
Step 2: "0-1" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "0-1" ("0" is read in; reading stops because the next character is a non-digit)
          ^

Example 5:

Input: s = "words and 987"

Output: 0

Explanation:

Reading stops at the first non-digit character 'w'.

Constraints:

0 <= s.length <= 200
s consists of English letters (lower-case and upper-case), digits (0-9), ' ', '+', '-', and '.'.

Solution

Clarifying Questions

When you get asked this question in a real-life environment, it will often be ambiguous (especially at FAANG). Make sure to ask these questions in that case:

What should I return if the input string is null or empty?
Besides leading whitespace, plus/minus signs, and digits, can the input string contain other characters? If so, how should I handle them?
Should I handle integer overflow or underflow by returning `Integer.MAX_VALUE` or `Integer.MIN_VALUE` respectively, or should I throw an exception?
Is it possible for there to be multiple plus or minus signs (e.g., "+-12")? If so, how should that be interpreted?
If the string contains non-numeric characters after the initial digits, should I stop parsing at the first non-numeric character or try to extract all valid integer sequences?

Brute Force Solution

Approach

The brute force approach involves examining the input string character by character and converting it to an integer if possible. We go through the string validating each character to determine if it can be part of an integer. We will handle all the edge cases step by step to meet the requirements.

Here's how the algorithm would work step-by-step:

First, remove any spaces at the beginning of the string.
Check if the first non-space character is a plus or minus sign. If it is, remember the sign; otherwise, assume it's positive.
Then, go through the remaining characters one by one.
If a character is a digit (0-9), add it to the result so far, building up the integer.
If a character is not a digit, stop and ignore the rest of the string.
Before returning the number, make sure it's within the valid range for integers (between a very large negative number and a very large positive number). If it's outside this range, adjust it to be the closest limit.
Finally, return the integer with the correct sign.

Code Implementation

def string_to_integer_atoi(input_string):
    string_index = 0
    string_length = len(input_string)

    # Remove leading whitespaces
    while string_index < string_length and input_string[string_index] == ' ':
        string_index += 1

    sign = 1
    if string_index < string_length:
        # Determine the sign
        if input_string[string_index] == '+':
            string_index += 1
        elif input_string[string_index] == '-':
            sign = -1
            string_index += 1

    result = 0
    while string_index < string_length and input_string[string_index].isdigit():
        digit = int(input_string[string_index])
        # Check for overflow before multiplying
        if result > (2**31 - 1) // 10 or (result == (2**31 - 1) // 10 and digit > 7):
            if sign == 1:
                return 2**31 - 1
            else:
                return -2**31

        result = result * 10 + digit
        string_index += 1

    # Apply the sign
    result = sign * result

    # Clamp the result to the 32-bit signed integer range
    if result > 2**31 - 1:
        return 2**31 - 1
    if result < -2**31:
        return -2**31

    return result

Big(O) Analysis

Time Complexity

O(n) – The algorithm iterates through the input string of length n at most once. The initial space removal takes at most n steps. Determining the sign takes constant time. The main loop iterates through the string, converting digits to an integer until a non-digit character is encountered or the end of the string is reached, which is bounded by n. Finally, the range checking and sign application take constant time. Therefore, the overall time complexity is dominated by the single pass through the string, resulting in O(n) time complexity.

Space Complexity

O(1) – The algorithm uses a constant amount of extra space. It stores a few variables: one to track the sign, one to accumulate the result, and potentially a few index variables to iterate through the string. The space used by these variables does not depend on the length of the input string N, so the space complexity is constant.

Optimal Solution

Approach

This problem asks us to convert a string into an integer. The optimal approach carefully parses the string, handling edge cases and potential errors along the way to avoid unnecessary calculations and ensure accuracy.

Here's how the algorithm would work step-by-step:

First, ignore any leading whitespace at the beginning of the string.
Check if there's a plus or minus sign indicating the number's sign. If present, remember it. If not, assume it's positive.
Start reading digits until you reach a non-digit character or the end of the string.
Convert the digits into an integer. Be careful of potential overflow (going above or below the maximum/minimum integer values). If an overflow happens, return the maximum or minimum integer value accordingly.
Apply the sign (positive or negative) that was determined earlier.
Return the resulting integer.

Code Implementation

def string_to_integer_atoi(input_string):
    string_index = 0
    string_length = len(input_string)

    # Remove leading whitespace.
    while string_index < string_length and input_string[string_index] == ' ':
        string_index += 1

    sign = 1
    # Determine the sign of the number.
    if string_index < string_length and (input_string[string_index] == '+' or input_string[string_index] == '-'):
        sign = -1 if input_string[string_index] == '-' else 1
        string_index += 1

    integer_result = 0

    # Convert digits to an integer.
    while string_index < string_length and input_string[string_index].isdigit():
        digit = int(input_string[string_index])

        # Check for potential overflow before updating result
        if integer_result > (2**31 - 1) // 10 or (integer_result == (2**31 - 1) // 10 and digit > 7):
            return 2**31 - 1 if sign == 1 else -2**31

        if integer_result < (-2)**31 // 10 or (integer_result == (-2)**31 // 10 and digit < -8):
            return 2**31 - 1 if sign == 1 else -2**31

        integer_result = integer_result * 10 + digit
        string_index += 1

    # Apply the sign.
    return sign * integer_result

Big(O) Analysis

Time Complexity

O(n) – The algorithm iterates through the input string once to identify leading whitespace, the sign, and subsequent digits. Each character is examined at most a constant number of times. The length of the string, denoted as 'n', determines the number of iterations. Therefore, the time complexity is directly proportional to the length of the input string, resulting in a linear time complexity.

Space Complexity

O(1) – The algorithm primarily uses a few variables to store the sign, current digit, and the resulting integer. The number of variables used is constant and does not depend on the length of the input string (N). Therefore, the auxiliary space required is constant.

Edge Cases

Case	How to Handle
Null or empty input string	Return 0 if the input string is null or empty.
String contains only whitespace characters	Return 0 if after trimming the string consists only of whitespace.
String starts with a non-numeric and non-sign character	Return 0 if the first non-whitespace character is not a digit or a sign (+/-).
Multiple sign characters at the beginning	Consider only the first valid sign character and ignore subsequent ones, potentially returning 0 if they alternate signs.
Integer overflow (positive)	Clamp the result to Integer.MAX_VALUE if the parsed integer exceeds the maximum positive 32-bit signed integer value.
Integer underflow (negative)	Clamp the result to Integer.MIN_VALUE if the parsed integer is less than the minimum negative 32-bit signed integer value.
String contains non-numeric characters after a valid integer	Stop parsing at the first non-numeric character after the optional sign and valid digits.
Leading zeros before significant digits	The leading zeros should be skipped during conversion but are still valid.