Introduction

The String to Integer (atoi) problem asks us to convert a string containing a number into an integer.

While converting, we need to correctly handle:

  • Leading spaces
  • Positive or negative signs
  • Digits
  • Non-digit characters
  • Integer overflow

For example:

" -42"

should produce:

-42

Problem Statement

Given a string, convert it into a 32-bit signed integer.

Follow these rules:

  1. Ignore leading spaces.
  2. Check for an optional + or - sign.
  3. Read consecutive digits.
  4. Stop when a non-digit character is encountered.
  5. If the value exceeds the 32-bit signed integer range, clamp it to the nearest limit.

Example

Input:"   -42"

Output:
-42

Another example:

Input:"4193 with words"

Output:
4193

Constraints

  • The input can contain letters, digits, spaces, and symbols.
  • The result must fit within the 32-bit signed integer range.
  • Minimum value: -2147483648
  • Maximum value: 2147483647

Approach 1: Brute Force

Explanation

A straightforward approach is to:

  1. Remove leading spaces.
  2. Extract the optional sign.
  3. Collect all consecutive digits.
  4. Convert the collected digits into an integer.
  5. Check whether the value is within the valid integer range.

This approach uses a temporary string to store the digits.

Steps

  1. Skip leading spaces.
  2. Check for + or -.
  3. Traverse the remaining characters.
  4. Stop when a non-digit character is found.
  5. Store the digits.
  6. Convert the digits into an integer.
  7. Handle overflow.

Dry Run

Input:"   -42abc"

Skip spaces:
"   -42abc"
   ↑

Sign:
-

Read digits:
4
2

Stop at:
a

Digits:
42

Apply sign:
-42

Output:
-42

Brute Force Code

Complexity Analysis

Time Complexity: O(n)

Space Complexity: O(n) because the digits are stored separately.

Approach 2: Optimized Solution

Explanation

We can avoid storing the digits separately.

Instead, build the number directly while traversing the string.

For every digit:

number = number × 10 + digit

Before adding a new digit, check whether doing so would cause overflow.

This allows us to process the string in a single pass.

Steps

  1. Skip leading spaces.
  2. Determine the sign.
  3. Initialize the number to 0.
  4. Read each digit.
  5. Check for overflow.
  6. Add the digit to the number.
  7. Stop at the first non-digit.
  8. Apply the sign.

Dry Run

Input:"   -42abc"
Skip spaces.

Sign:
-

Start:
number = 0

Read 4:
number = 0 × 10 + 4
number = 4

Read 2:
number = 4 × 10 + 2
number = 42

Next character:
a
Stop.

Apply sign:
42 × -1 = -42

Output:
-42

Optimized Code

Complexity Analysis

Time Complexity: O(n)

Space Complexity: O(1)

Edge Cases

  • "42" → 42
  • " -42" → -42
  • "+42" → 42
  • "4193 with words" → 4193
  • "words 42" → 0
  • "000123" → 123
  • Very large positive number → 2147483647
  • Very large negative number → -2147483648
  • Empty string → 0

Why This Problem is Important

This problem teaches how to carefully process strings while handling:

  • Whitespace
  • Signs
  • Digits
  • Invalid characters
  • Integer overflow

It is a common interview problem because it tests edge-case handling and implementation accuracy.

Real-World Applications

String-to-number conversion is commonly used in:

  • Processing user input
  • Reading configuration values
  • Parsing command-line arguments
  • Processing CSV and text data
  • Converting form input into numeric values

Common Mistakes

  1. Not skipping leading spaces.
  2. Accepting multiple signs.
  3. Continuing after encountering a non-digit.
  4. Forgetting to handle negative numbers.
  5. Ignoring integer overflow.
  6. Using a data type that cannot safely hold intermediate values.

Interview Tips

  • Explain the parsing rules before writing code.
  • Handle spaces first.
  • Process the sign before digits.
  • Stop immediately at the first non-digit.
  • Always discuss 32-bit integer overflow.
  • Avoid relying entirely on a built-in conversion function if the interviewer asks for implementation.

Related Questions

  • String to Integer Conversion
  • Valid Number
  • Reverse Integer
  • Add Strings
  • Multiply Strings
  • Compare Version Numbers

Final Takeaway

The String to Integer (atoi) problem requires careful parsing of a string.

The optimized solution processes the input in one pass, builds the number digit by digit, and handles signs and overflow efficiently.

Time Complexity: O(n)

Space Complexity: O(1)