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7. Plus One

easyAsked at Etsy

Add one to a number represented as a digit array — Etsy uses it to test carry-handling and overflow edge cases.

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

Problem

Given a non-empty array of decimal digits representing a non-negative integer, increment the integer by one and return the resulting digit array. The most significant digit is at the head.

Constraints

  • 1 <= digits.length <= 100
  • 0 <= digits[i] <= 9
  • No leading zeros except for the number 0 itself

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 via BigInt

Join digits to a string, BigInt + 1, split back.

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, propagate the carry, prepend 1 if it survives.

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:

Etsy-specific tips

Etsy weights edge-case discipline highly — explicitly call out the all-nines case before you start coding to earn the bonus signal.

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Output

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