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7. Plus One

easyAsked at Asana

Given a number represented as a digit array, return the array plus one. Asana uses this to gauge whether you handle carry propagation cleanly — the same pattern that shows up in their counter-aggregation services.

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

Source citations

Public interview reports confirming this problem appears in Asana loops.

  • Glassdoor (2026-Q1)— Asana new-grad screen warmup.

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

Example 3

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

Approaches

1. Convert to number, add one, back to digits

Parse the array, add one, split back.

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

Tradeoff: Requires BigInt for 100-digit numbers. Asana wants the carry logic in-place.

2. In-place carry propagation

Walk right-to-left, add one, carry while you hit a 9.

Time
O(n)
Space
O(1) extra
function plusOne(d) {
  for (let i = d.length - 1; i >= 0; i--) {
    if (d[i] < 9) { d[i]++; return d; }
    d[i] = 0;
  }
  // all 9's: need to prepend 1
  return [1, ...d];
}

Tradeoff: Early return on the first non-9 digit. The all-9s case is the only allocation.

Asana-specific tips

Asana wants you to handle the all-9s edge case cleanly — the early-return pattern beats the 'always carry' version because it short-circuits. Mention that the same carry pattern shows up in their counter services and you'll get bonus signal.

Common mistakes

  • Forgetting the all-9s case (e.g., [9,9,9] -> [1,0,0,0]).
  • Iterating left-to-right instead of right-to-left.
  • Mutating during iteration in a way that breaks the loop bound.

Follow-up questions

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

  • Add two numbers represented as arrays (LC 989).
  • Multiply two numbers represented as strings (LC 43).
  • What if the digit base isn't 10?

Solve it now

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Output

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FAQ

Why right-to-left?

Because carries propagate from least to most significant. Going left-to-right would force you to look ahead for carries, which is harder to write correctly.

Can you avoid the prepend allocation?

Only if you can resize the input array in-place. In JS, [1, ...d] is the cleanest; in C, you'd realloc.

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