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Optimal Lab

Strings · 100Medium

String to Integer (atoi)

Implement atoi: skip leading whitespace, read an optional '+' or '-' sign, then read as many digits as possible and stop at the first non-digit. Clamp the result to the 32-bit signed range [-2^31, 2^31 - 1]. Return 0 if no digits.

Constraints

  • 0 <= s.length <= 200
  • s consists of letters, digits, ' ', '+', '-' and '.'.

Examples

myAtoi("42")               ->  42
myAtoi("   -42")           ->  -42
myAtoi("4193 with words")  ->  4193
myAtoi("words and 987")    ->  0      // stops at the first non-digit
myAtoi("")                 ->  0      // no digits at all
myAtoi("-91283472332")     ->  -2147483648   // clamped to -2^31

Edge cases

  • Only LEADING whitespace is skipped; whitespace after digits ends the number.
  • At most one sign character, and it must come before the digits.
  • Out-of-range values clamp to the 32-bit bound rather than wrapping.
Show the target complexity (spoiler)
Naive
parseInt gets most cases right but does not clamp to the 32-bit range.
Target
O(n) time, O(1) space — a small state machine: whitespace, then sign, then digits, clamping as you accumulate.

Solve it

By hand, in your editor. The runner checks answers and timing.

git clone https://github.com/sarmadkung/optimal-round
cd optimal-round
# open problems/15-strings/100-string-to-integer-atoi.js
./practice c 100
Open the file on GitHub ↗