DSA interview question

String to Integer (atoi)

Medium · Algorithms · Topics: string, math

The problem

A configuration loader reads user-typed numbers such as " -42", "4193 with words" or "99999999999" and must turn them into 32-bit integers the way C's atoi does. Skip leading spaces, read an optional sign, read digits until the first non-digit, and clamp anything outside the 32-bit range to INT_MIN or INT_MAX. The parsing is easy; the part interviewers watch is detecting overflow before it happens, without relying on a 64-bit type.

Example: " -42" → -42; "-91283472332" → -2147483648.

Function signature

int myAtoi(const string& s);  // spaces, optional sign, digits; clamped to the int range

Test cases

Input `s`	Output	Explanation
`"42"`	42
`" -42"`	-42	Leading spaces and a sign
`"4193 with words"`	4193	Stops at the first non-digit
`"words and 987"`	0	No digits before a letter
`"-91283472332"`	-2147483648	Clamped to INT_MIN
`"2147483648"`	2147483647	Clamped to INT_MAX
`"+-12"`	0	Only one sign is allowed
`" 0000000000012345678"`	12345678	Leading zeros are fine
`""`	0	Empty input

(Confirm: only the space character counts as whitespace; the sign must directly precede the digits; out-of-range values clamp.)

How interviewers phrase it

"Implement atoi, which converts a string to a 32-bit signed integer" (LeetCode 8).
A test of careful edge-case handling and overflow detection rather than algorithmic cleverness.

Clarifying questions to ask

Which characters count as whitespace?
What happens on overflow: clamp, or return 0?
Is "+" or "-" alone valid? (It gives 0.)
Can I use a 64-bit integer internally?

Follow-ups to expect

"How do you detect overflow without 64-bit arithmetic?"
"Does clamping to INT_MAX handle negatives correctly?"
"Why stop at the first non-digit?"
"What about a 64-bit accumulator?"
"Complexity?"

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