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7. Plus One

easyAsked at DigitalOcean

Increment a non-negative integer represented as a digit array — DigitalOcean uses this to test boundary handling that resembles incrementing version counters on droplet snapshot IDs.

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 leading digit is the most significant.

Constraints

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

Examples

Example 1

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

Example 2

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

Approaches

1. BigInt parse

Convert to BigInt, add 1, stringify 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 carry, prepend 1 if final carry remains.

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:

DigitalOcean-specific tips

DigitalOcean expects the carry-propagation answer because string conversion hides cost at the scale of per-region usage counters.

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Output

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