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7. Plus One

easyAsked at Figma

Given a non-empty array representing a non-negative integer, increment the number by one. Figma uses this as a 5-minute warm-up to gauge whether candidates handle carry propagation cleanly, the same pattern as their CRDT version-vector increments.

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

Source citations

Public interview reports confirming this problem appears in Figma loops.

  • LeetCode Discuss (2025-08)— Figma OA easy bucket.
  • Glassdoor (2026-Q1)— Warm-up before a longer canvas problem.

Problem

You are given a large integer represented as an integer array digits, where each digits[i] is the i-th digit of the integer. The digits are ordered from most significant to least significant in left-to-right order. The large integer does not contain any leading 0's. 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.

Examples

Example 1

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

Example 2

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

Approaches

1. Convert to integer

Parse the digits as a number, add 1, split back to digits.

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

Tradeoff: Works only with BigInt because regular numbers overflow at 100 digits. Cute but misses the point.

2. In-place carry propagation

Walk right-to-left; add 1 with carry; if every digit was 9, prepend a leading 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: Linear, constant extra. Carries only happen when the digit is 9 — early exit otherwise.

Figma-specific tips

Figma uses Plus One to gauge whether you can think about carry propagation cleanly — exactly the mental model their version-vector / Lamport-timestamp increments need. State the invariant 'I either increment and stop, or set to 0 and continue' before writing the loop; that explicit invariant separates competent from confused candidates.

Common mistakes

  • Treating digits as a number directly — overflows for n >= 16 digits.
  • Forgetting the all-9s case (need to prepend 1).
  • Off-by-one when iterating from the wrong end.

Follow-up questions

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

  • Add Two Numbers (LC 2) — linked-list version of carry.
  • Add Binary (LC 67) — binary variant.
  • Multiply Strings (LC 43) — generalize to full arithmetic.

Solve it now

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Output

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FAQ

Do I need BigInt?

Only if you go the parse-to-number route. The carry-propagation approach never touches a numeric type, so it scales to any length.

What's the all-9s edge case?

[9,9,9] + 1 = [1,0,0,0]. The loop sets each digit to 0 and falls out without returning, so you prepend a leading 1 at the end.

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