Problem 8: String to Integer (atoi)

Difficulty: Medium
Topics: String
LeetCode Link: Problem URL

Problem Description

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 '.'.

Approach

This problem requires converting a string to a 32-bit signed integer, handling leading whitespace, signedness, and potential overflow. The solution involves iterating through the string while applying the rules specified in the problem statement.

Steps

Skip leading whitespace characters.
Check for a sign character ('-' or '+') and set the sign accordingly.
Read digits until a non-digit character is encountered, constructing the integer.
Handle overflow by checking if the number exceeds the 32-bit signed integer limits.
Return the final integer value, applying the sign.

Solution

Language: C++
File: solution.cpp

Language: Java
File: solution.java

Notes

Ensure to handle edge cases such as leading zeros, non-digit characters, and overflow conditions.

Complexity Analysis

Time Complexity: O(n), where n is the length of the string s. The algorithm processes each character at most once.
Space Complexity: O(1), as we are using a constant amount of space for variables.

Test Cases

Some test cases to validate the solution:

Running Tests

C++ (Simple): cd tests; g++ test.cpp ../solution.cpp -o test; ./test
Python: cd tests; python test.py
Java: javac .\solution.java tests/test.java; java -cp ".;tests" test

Solved on: 14/06/2025 | Time taken: 00:15:11 | Attempts: 5