7. Plus One

easyAsked at Canva

Increment a number represented as a digit array — Canva uses this to gauge how you handle carries and array growth, similar to incrementing canvas page numbers.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-20

Problem

Given a non-empty array of decimal digits representing a non-negative integer, increment by one. Digits are stored most-significant first; modify and return the result array.

Constraints

1 <= digits.length <= 100
0 <= digits[i] <= 9
No leading zeros

Examples

Example 1

Input

digits = [1,2,3]

Output

[1,2,4]

Example 2

Input

digits = [9,9]

Output

[1,0,0]

Approaches

1. BigInt convert

Parse to BigInt, add 1, split back to digits.

Time: O(n)
Space: O(n)

const n = BigInt(digits.join('')) + 1n;
return String(n).split('').map(Number);

Tradeoff:

2. In-place carry

Walk right to left propagating the carry; only grow the array if every digit was 9.

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;
  }
  digits.unshift(1);
  return digits;
}

Tradeoff:

Canva-specific tips

Canva graders look for clean carry logic over flashy BigInt tricks, mirroring how their backend handles overflow-safe counters across the design corpus.

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Output

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