String to Integer atoi in DSA Python - Time & Space Complexity

Time Complexity: String to Integer atoi

O(n)

Understanding Time Complexity

We want to understand how the time taken to convert a string to an integer grows as the string gets longer.

Specifically, how does the number of steps change when the input string size increases?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

def myAtoi(s: str) -> int:
    i, n = 0, len(s)
    while i < n and s[i] == ' ':
        i += 1
    sign = 1
    if i < n and s[i] in ['+', '-']:
        sign = -1 if s[i] == '-' else 1
        i += 1
    result = 0
    while i < n and s[i].isdigit():
        result = result * 10 + (ord(s[i]) - ord('0'))
        i += 1
    return sign * result
    

This code converts a string to an integer by skipping spaces, handling sign, and reading digits.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Two while loops scanning characters in the string.
• How many times: Each loop runs at most once over the string characters, total scanning each character at most once.

How Execution Grows With Input

The code scans the string from start to end, checking characters one by one.

Input Size (n)	Approx. Operations
10	About 10 character checks
100	About 100 character checks
1000	About 1000 character checks

Pattern observation: The number of steps grows roughly in direct proportion to the string length.

Final Time Complexity

Time Complexity: O(n)

This means the time to convert the string grows linearly with the length of the input string.

Common Mistake

[X] Wrong: "The time complexity is constant because we only read until the first non-digit character."

[OK] Correct: In the worst case, the entire string can be digits, so the code must check every character, making time grow with input size.

Interview Connect

Understanding this linear time complexity helps you explain how your solution scales with input size, a key skill in interviews.

Self-Check

"What if the input string was given as a linked list of characters instead of a string? How would the time complexity change?"

Practice

1. What does the atoi function do when given a string input?

easy

A. Converts the string to an integer number

B. Converts the string to a floating-point number

C. Reverses the string characters

D. Counts the number of characters in the string

Solution

1. Step 1: Understand the purpose of atoi

   The atoi function is designed to convert a string that represents a number into an integer type.

2. Step 2: Compare options with function behavior

   Only Converts the string to an integer number matches the behavior of converting string to integer. Others describe different operations.

3. Final Answer:

   Converts the string to an integer number -> Option A

4. Quick Check:

   atoi = string to integer [OK]

Hint: Remember atoi means ASCII to integer conversion [OK]

Common Mistakes:

• Thinking atoi converts to float
• Confusing atoi with string reversal
• Assuming atoi counts characters

2. Which of the following Python code snippets correctly converts a string s to an integer similar to atoi?

easy

A. len(s)

B. int(s)

C. float(s)

D. str(s)

Solution

1. Step 1: Identify Python function for string to integer

   Python's built-in int() function converts a string representing a number into an integer.

2. Step 2: Check other options

   str() converts to string, float() converts to float, and len() returns length, so they are incorrect.

3. Final Answer:

   int(s) -> Option B

4. Quick Check:

   int() converts string to integer [OK]

Hint: Use int() to convert string to integer in Python [OK]

Common Mistakes:

• Using str() instead of int()
• Using float() when integer is needed
• Using len() expecting number conversion

3. What will be the output of the following Python code?

def my_atoi(s):
    s = s.strip()
    sign = 1
    if s and s[0] == '-':
        sign = -1
        s = s[1:]
    num = 0
    for char in s:
        if not char.isdigit():
            break
        num = num * 10 + (ord(char) - ord('0'))
    return sign * num

print(my_atoi('  -123abc'))

medium

A. 0

B. 123

C. -123

D. Error

Solution

1. Step 1: Analyze string trimming and sign detection

   The string ' -123abc' is stripped to '-123abc'. The sign is detected as negative and removed from string, leaving '123abc'.

2. Step 2: Convert digits until non-digit character

   Loop reads '1', '2', '3' converting to number 123. Stops at 'a' which is not digit.

3. Final Answer:

   -123 -> Option C

4. Quick Check:

   Parsed number with sign = -123 [OK]

Hint: Stop parsing at first non-digit character [OK]

Common Mistakes:

• Ignoring sign and returning positive number
• Parsing entire string including letters
• Raising error on non-digit characters

4. Find the bug in this atoi implementation and choose the correct fix:

def atoi_buggy(s):
    s = s.strip()
    sign = 1
    if s[0] == '-':
        sign = -1
        s = s[1:]
    num = 0
    for char in s:
        if not char.isdigit():
            return 0
        num = num * 10 + int(char)
    return sign * num

print(atoi_buggy('  -42abc'))

medium

A. Add check if string is empty before accessing s[0]

B. Change return 0 to break inside loop

C. Remove sign variable and always return positive

D. Replace int(char) with ord(char)

Solution

1. Step 1: Identify potential error with empty string

   If input is empty or only spaces, s[0] access causes IndexError. Need to check if s is not empty before accessing.

2. Step 2: Evaluate other fixes

   Changing return 0 to break is good but does not fix crash on empty string. Removing sign loses negative support. Using ord(char) is incorrect for digit conversion.

3. Final Answer:

   Add check if string is empty before accessing s[0] -> Option A

4. Quick Check:

   Check empty string before indexing [OK]

Hint: Always check string length before indexing [OK]

Common Mistakes:

• Not checking empty string before s[0]
• Returning 0 instead of stopping parse
• Ignoring negative sign handling

5. You want to implement atoi that handles leading spaces, optional sign, digits, and clamps output to 32-bit signed integer range [-2^31, 2^31-1]. Which approach correctly applies this?

hard

A. Convert entire string to int directly and then clamp

B. Return 0 if string contains any non-digit characters

C. Ignore sign and clamp only positive values

D. Parse string, convert digits, apply sign, then clamp result between -2147483648 and 2147483647

Solution

1. Step 1: Understand requirements for atoi with clamping

   We must parse string carefully: skip spaces, detect sign, convert digits until non-digit, then clamp to 32-bit integer range.

2. Step 2: Evaluate options for correctness

   Parse string, convert digits, apply sign, then clamp result between -2147483648 and 2147483647 correctly describes parsing and clamping. Convert entire string to int directly and then clamp risks error if string has invalid chars. Ignore sign and clamp only positive values ignores sign which is wrong. Return 0 if string contains any non-digit characters returns 0 too aggressively.

3. Final Answer:

   Parse string, convert digits, apply sign, then clamp result between -2147483648 and 2147483647 -> Option D

4. Quick Check:

   Parse, sign, clamp = correct atoi [OK]

Hint: Clamp after parsing signed integer to avoid overflow [OK]

Common Mistakes:

• Clamping before applying sign
• Not handling non-digit characters properly
• Ignoring integer overflow limits