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8. Plus One

easyAsked at Workday

Given a non-negative integer as a digit array, increment by one and return as a digit array. Workday uses this to test carry-propagation discipline — payroll batch IDs are big integers that increment past JS number precision.

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

Source citations

Public interview reports confirming this problem appears in Workday loops.

  • Glassdoor (2025)— Workday SDE1 screen — bigint 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, convert back

BigInt(digits.join('')) + 1n, then split back.

Time
O(n)
Space
O(n)
return [...(BigInt(digits.join('')) + 1n).toString()].map(Number);

Tradeoff: Defeats the purpose — interviewer wants you to handle carry manually.

2. Right-to-left carry

Walk from least significant. If digit < 9, increment and return. Otherwise zero out and carry. If we fall off the front, prepend 1.

Time
O(n)
Space
O(1) extra (output reuses input)
function plusOne(digits) {
  for (let i = digits.length - 1; i >= 0; i--) {
    if (digits[i] < 9) {
      digits[i]++;
      return digits;
    }
    digits[i] = 0;
  }
  // Every digit was 9 — e.g., [9,9,9] -> [1,0,0,0]
  return [1, ...digits];
}

Tradeoff: Single pass, early exit on the first non-9. Only allocates a new array in the all-9s edge case.

Workday-specific tips

Workday grades the [9,9,9] edge case. Many candidates forget to prepend 1 and return [0,0,0]. Walking through this case before coding earns trust. Mention the payroll-batch-ID analogy if the interviewer asks about real-world relevance.

Common mistakes

  • Converting to Number — overflows past 15 digits.
  • Forgetting the all-9s case (no early return triggered).
  • Using digits.unshift(1) — fine semantically but O(n); the [1, ...digits] form makes the allocation visible.

Follow-up questions

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

  • Add Binary (LC 67) — same pattern, base 2.
  • Add Strings (LC 415) — arbitrary-length integer addition.
  • Multiply Strings (LC 43) — extend to multiplication.

Solve it now

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Output

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FAQ

Why not use BigInt?

The interviewer is testing whether you can implement carry manually. BigInt sidesteps the problem entirely.

Why right-to-left?

Carry propagates from least significant to most significant — same as how you'd add by hand.

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