Skip to main content

8. Plus One

easyAsked at Reddit

Add one to a big integer represented as an array of digits. Reddit asks this to test careful carry-handling — the same kind of mental model needed for monotonic karma counters that survive past 2^31.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Source citations

Public interview reports confirming this problem appears in Reddit loops.

  • Glassdoor (2026-Q1)— Reddit infra team uses this as a 5-minute warm-up before harder distributed-counter questions.

Problem

You are given a large integer represented as an integer array digits, where each digits[i] is the ith digit of the integer. The digits are ordered from most significant to least significant. Increment the large integer by one and return the resulting array of digits.

Constraints

  • 1 <= digits.length <= 100
  • 0 <= digits[i] <= 9
  • digits does not contain any leading 0's, except for the number 0 itself.

Examples

Example 1

Input
digits = [1,2,3]
Output
[1,2,4]

Example 2

Input
digits = [4,3,2,1]
Output
[4,3,2,2]

Example 3

Input
digits = [9]
Output
[1,0]

Approaches

1. Convert to BigInt, add 1, convert back

Join digits to string, parse to BigInt, add, split back.

Time
O(n)
Space
O(n)
function plusOne(digits) {
  const n = BigInt(digits.join('')) + 1n;
  return n.toString().split('').map(Number);
}

Tradeoff: Works but defeats the purpose. Interviewers flag this as 'avoiding the problem'.

2. In-place carry from right (optimal)

Walk from the last digit. If 9, set to 0 and continue. Otherwise increment and return. If we fall off the left, prepend 1.

Time
O(n)
Space
O(1)
function plusOne(digits) {
  for (let i = digits.length - 1; i >= 0; i--) {
    if (digits[i] < 9) {
      digits[i]++;
      return digits;
    }
    digits[i] = 0;
  }
  return [1, ...digits];
}

Tradeoff: Single pass, O(1) extra (other than the all-9s case).

Reddit-specific tips

Reddit interviewers expect you to ask: 'can we mutate the input?' If yes, the in-place version is ideal. Bonus signal: mention how their karma counter would face this exact problem if stored as a base-10 array for cross-shard merging.

Common mistakes

  • Forgetting the all-9s case (returns [1, 0, 0, ...]).
  • Using parseInt — overflows for digits.length > 16.
  • Iterating left-to-right (carry direction is right-to-left).

Follow-up questions

An interviewer at Reddit may pivot to one of these next:

  • Add two big integers represented as arrays (LC 415).
  • Add binary strings (LC 67).
  • Multiply two big integers as strings (LC 43).

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

FAQ

Why not use Number.parseInt?

JavaScript's Number is a 64-bit float — only safe up to 2^53 - 1. The constraint allows 100 digits.

Could we use Array.prototype.unshift instead of spread?

Yes, but unshift is O(n) anyway. Spread is clearer.

Practice these live with InterviewChamp.AI

Drill Plus One and other Reddit interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →