10.2 String to Integer (atoi)

10.2.1 Problem Metadata

10.2.2 Description

Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer (similar to C/C++’s atoi function).

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

  1. Read in and ignore any leading whitespace.
  2. Check if the next character (if not already at the end of the string) is ‘-’ or ‘+’. Read this character in if it is either. This determines if the final result is negative or positive respectively. Assume the result is positive if neither is present.
  3. Read in next the characters until the next non-digit character or the end of the input is reached. The rest of the string is ignored.
  4. Convert these digits into an integer (i.e., "123" → 123, "0032" → 32). If no digits were read, then the integer is 0. Change the sign as necessary (from step 2).
  5. If the integer is out of the 32-bit signed integer range \([-2^{31}, 2^{31} - 1]\), then clamp the integer so that it remains in the range. Specifically, integers less than \(-2^{31}\) should be clamped to \(-2^{31}\), and integers greater than \(2^{31} - 1\) should be clamped to \(2^{31} - 1\).
  6. Return the integer as the final result.

Note: - Only the space character ' ' is considered a whitespace character. - Do not ignore any characters other than the leading whitespace or the rest of the string after the digits.

10.2.3 Examples

Input: s = "42"
Output: 42
Explanation: The underlined characters are what is read in, 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)
The parsed integer is 42.
Since 42 is in the range [-2^31, 2^31 - 1], the final result is 42.

Input: s = "   -42"
Output: -42
Explanation:
Step 1: "   -42" (leading whitespace is read and ignored)
Step 2: "   -42" ('-' is read, so the result should be negative)
Step 3: "   -42" ("42" is read in)
The parsed integer is -42.
Since -42 is in the range [-2^31, 2^31 - 1], the final result is -42.

Input: s = "4193 with words"
Output: 4193
Explanation:
Step 1: "4193 with words" (no characters read because there is no leading whitespace)
Step 2: "4193 with words" (no characters read because there is neither a '-' nor '+')
Step 3: "4193 with words" ("4193" is read in; reading stops because the next character is a non-digit)
The parsed integer is 4193.
Since 4193 is in the range [-2^31, 2^31 - 1], the final result is 4193.

Input: s = "words and 987"
Output: 0
Explanation:
Step 1: "words and 987" (no characters read because there is no leading whitespace)
Step 2: "words and 987" (no characters read because there is neither a '-' nor '+')
Step 3: "words and 987" (reading stops immediately because there is a non-digit 'w')
The parsed integer is 0 because no digits were read.
Since 0 is in the range [-2^31, 2^31 - 1], the final result is 0.

Input: s = "-91283472332"
Output: -2147483648
Explanation:
Step 1: "-91283472332" (no characters read because there is no leading whitespace)
Step 2: "-91283472332" ('-' is read, so the result should be negative)
Step 3: "-91283472332" ("91283472332" is read in)
The parsed integer is -91283472332.
Since -91283472332 is less than the lower bound of the range [-2^31, 2^31 - 1], the final result is clamped to -2^31 = -2147483648.

10.2.4 Constraints

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

10.2.5 Solution - Character-by-Character Parsing

10.2.5.1 Walkthrough

This solution implements a straightforward character-by-character parsing approach to convert a string to an integer following the atoi specification. The algorithm uses a state machine-like pattern with boolean flags to track the parsing state.

Core Strategy:

The solution handles the conversion in a single pass through the trimmed string, maintaining two critical pieces of state: - isStart: Whether we’ve begun reading the number (either by seeing a sign or a digit) - isNegative: Whether the number should be negative

Key Design Decisions:

  1. Use long for overflow detection: By using long result, we can safely build the number and detect when it exceeds Integer.MAX_VALUE before clamping
  2. Trim first: Calling s.trim() eliminates all leading and trailing whitespace in one operation
  3. Conditional logic flow: The series of if-else blocks creates a priority-based state machine

Step-by-Step Algorithm:

After trimming, for each character we check (in priority order):

  1. Is it a digit? → Build the result, set isStart = true
  2. Is it a ‘-’ sign? → Only valid if !isStart, sets isNegative = true and isStart = true
  3. Is it a ‘+’ sign? → Only valid if !isStart, sets isNegative = false and isStart = true
  4. Is it a space or dot? → Immediately break (stops parsing)
  5. Is isStart already true? → Non-digit after digits, break
  6. Otherwise → Invalid leading character (like ‘w’ in “words and 987”), break

Why This Works:

The key insight is that after trim(), the string has NO leading/trailing spaces. So: - Any space encountered during iteration is a mid-string space that should stop parsing - The sign checks prevent multiple signs like "+-12" or signs after digits like "12-3" - The final else block catches invalid leading characters before any digits are read

Visual Example: s = "4193 with words"

After trim: "4193 with words"

Iteration 1: c='4'
  isDigit? YES → result = 0*10 + 4 = 4, isStart = true

Iteration 2: c='1'
  isDigit? YES → result = 4*10 + 1 = 41

Iteration 3: c='9'
  isDigit? YES → result = 41*10 + 9 = 419

Iteration 4: c='3'
  isDigit? YES → result = 419*10 + 3 = 4193

Iteration 5: c=' '
  isDigit? NO
  c=='-'? NO
  c=='+'? NO
  c==' ' || c=='.'? YES → break

Final: result = 4193, isNegative = false
Return: 4193 ✓

Visual Example: s = "words and 987"

After trim: "words and 987"

Iteration 1: c='w'
  isDigit? NO
  c=='-'? NO
  c=='+'? NO
  c==' ' || c=='.'? NO
  isStart? NO (still false)
  else → break (invalid leading character)

Final: result = 0, isNegative = false
Return: 0 ✓

Visual Example: s = " -42"

After trim: "-42"

Iteration 1: c='-'
  isDigit? NO
  c=='-'? YES
    isStart? NO → isNegative = true, isStart = true

Iteration 2: c='4'
  isDigit? YES → result = 0*10 + 4 = 4, isStart = true

Iteration 3: c='2'
  isDigit? YES → result = 4*10 + 2 = 42

Final: result = 42, isNegative = true
Return: 42 * -1 = -42 ✓

Visual Example: s = "-91283472332" (Overflow)

After trim: "-91283472332"

Iteration 1: c='-'
  c=='-'? YES → isNegative = true, isStart = true

Iterations 2-12: Build result digit by digit
  result = 9 → 91 → 912 → 9128 → 91283 → 912834 → 9128347 → 91283472 → 912834723 → 9128347233 → 91283472332

  After iteration 12: result = 91283472332 > Integer.MAX_VALUE (2147483647)

Final overflow check:
  result > Integer.MAX_VALUE? YES
  isNegative? YES → return Integer.MIN_VALUE = -2147483648 ✓

Edge Cases Handled:

  1. Leading whitespace: " 42" → trim removes spaces → "42" → 42 ✓
  2. Sign only: "-" → sign sets isStart=true, no digits read → result=0 → 0 ✓
  3. Multiple signs: "+-12" → ‘+’ sets isStart=true, ‘-’ sees isStart=true → breaks → result=0 → 0 ✓
  4. Dot character: "3.14" → reads ‘3’, encounters ‘.’ → breaks → 3 ✓
  5. Sign after digits: "12-3" → reads “12”, encounters ‘-’ with isStart=true → breaks → 12 ✓

10.2.5.2 Analysis

  • Time Complexity: O(n) where n is the length of the string
    • trim(): O(n) to scan for leading/trailing whitespace
    • Single pass through characters: O(n)
    • Each character is processed once with O(1) operations
    • Total: O(n)
  • Space Complexity: O(n) for the trimmed string, or O(1) if we don’t count the trimmed string
    • s.trim() creates a new string: O(n)
    • All other variables (result, flags): O(1)
    • If we avoid trim() and handle whitespace in the loop, space becomes O(1)

10.2.5.3 Implementation Steps

  1. Initialize state variables:

    • long result = 0 to safely detect overflow
    • boolean isStart = false to track if we’ve begun reading the number
    • boolean isNegative = false to track sign

  2. Trim the input string using s.trim() to remove leading/trailing whitespace

  3. Iterate through each character in the trimmed string:

    • If digit:
      • Check for overflow before updating (early exit optimization)
      • Set isStart = true
      • Update result = result * 10 + (c - '0')
    • Else if ‘-’ sign:
      • If isStart is already true, break (sign after digits/sign)
      • Otherwise, set isNegative = true and isStart = true
    • Else if ‘+’ sign:
      • If isStart is already true, break (sign after digits/sign)
      • Otherwise, set isNegative = false and isStart = true
    • Else if space or dot:
      • Break immediately (stops parsing)
    • Else if isStart is true:
      • Non-digit character after reading digits, break
    • Else:
      • Invalid leading character before any digits, break

  4. Handle overflow after the loop:

    • If result > Integer.MAX_VALUE:
      • Return Integer.MIN_VALUE if negative
      • Return Integer.MAX_VALUE if positive

  5. Apply sign and return:

    • If isNegative, return (int) result * -1
    • Otherwise, return (int) result

10.2.5.4 Code - Java

public int myAtoi(String s) {
    long result = 0;
    boolean isStart = false;
    boolean isNegative = false;

    // Trim the leading and trailing whitespace
    s = s.trim();

    for (char c : s.toCharArray()) {
        if (Character.isDigit(c)) {
            // Check for overflow before multiplication
            if (result > Integer.MAX_VALUE) {
                break;
            }

            isStart = true;
            result = result * 10 + (c - '0');
        } else if (c == '-') {
            if (isStart) {
                break;  // Sign after digits or another sign
            } else {
                isNegative = true;
                isStart = true;
            }
        } else if (c == '+') {
            if (isStart) {
                break;  // Sign after digits or another sign
            } else {
                isNegative = false;
                isStart = true;
            }
        } else if (c == ' ' || c == '.') {
            // Break once it reads a non-leading whitespace or dot
            break;
        } else if (isStart) {
            // Reading ends after some digits were read
            break;
        } else {
            // Invalid leading character (e.g., 'w' in "words and 987")
            break;
        }
    }

    // Clamp to 32-bit signed integer range
    if (result > Integer.MAX_VALUE) {
        if (isNegative) {
            return Integer.MIN_VALUE;
        } else {
            return Integer.MAX_VALUE;
        }
    }

    if (isNegative) {
        return (int) result * -1;
    } else {
        return (int) result;
    }
}