String to Integer (atoi)
String to Integer (atoi) written straight from the problem statement, then rewritten as a three state deterministic finite automaton.
Updated July 30, 2026
Original: LeetCode 8. String to Integer (atoi)
Difficulty: Medium
Topics: String
Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer (similar to C/C++'s atoifunction).
The algorithm for myAtoi(string s) is as follows:
- Read in and ignore any leading whitespace.
- 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.
- 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.
- 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).
- 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 -231 should be clamped to -231, and integers greater than 231 - 1 should be clamped to 231 - 1.
- 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.
Example 1:
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 [-231, 231 - 1], the final result is 42.Example 2:
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 [-231, 231 - 1], the final result is -42.Example 3:
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 [-231, 231 - 1], the final result is 4193.Constraints:
- 0 <= s.length <= 200
- s consists of English letters (lower-case and upper-case), digits (0-9), ' ', '+', '-', and '.'.
Solution
Brute force
Approach: not much to say. The problem statement spells out every step.
The boundary handling is the same job as "Reverse Integer". The difference is that this one clamps the result into range while that one returns 0 the moment it overflows.
Complexity: O(n)
class Solution:
def myAtoi(self, s: str) -> int:
length = len(s)
res, idx = 0, 0
is_positive = True
# remove space
while idx < length and s[idx].isspace():
idx = idx + 1
# determine positive or negative
if idx < length and (s[idx] == '+' or s[idx] == '-'):
is_positive = False if s[idx] == '-' else True
idx = idx + 1
# calculate
while idx < length and s[idx] >= '0' and s[idx] <= '9':
res = res * 10 + int(s[idx])
idx = idx + 1
# boundary check
return min(res, 2**31 - 1) if is_positive else max(-res, -2**31)Result: 35ms, beats 86.31% of users with Python3.
Deterministic finite automaton (DFA)
Approach: the steps in the problem statement split cleanly into three states:
Implementing that DFA is the whole solution.
The brute force in "Zigzag Conversion" also uses a state variable to control the read direction, just without ever becoming a full state machine.
Complexity: O(n)
class Solution:
def myAtoi(self, s: str) -> int:
length = len(s)
res, idx, state = 0, 0, 0
is_positive = True
if length == 0:
return 0
for current in s:
if state == 0:
if current == ' ':
continue
if current == '+' or current == '-':
is_positive = current != '-'
state = 1
continue
if current.isdigit():
state = 2
res = res * 10 + int(current)
continue
return 0
if state == 1:
if current.isdigit():
state = 2
res = res * 10 + int(current)
continue
return 0
if state == 2:
if current.isdigit():
res = res * 10 + int(current)
else:
break
return min(res, 2**31 - 1) if is_positive else max(-res, -2**31)Result: 29ms, beats 97.75% of users with Python3.