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7. Plus One

easyAsked at Dropbox

Increment a big-integer represented as a digit array; Dropbox uses it to probe carry-propagation logic for monotonically increasing version counters.

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

Problem

Given a non-empty digit array representing a non-negative integer (most-significant digit first), increment the integer by one. Return the resulting digit array.

Constraints

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

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 cast

Convert to BigInt, add one, split back into digits.

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

Tradeoff:

2. Reverse carry-walk

Walk from least-significant digit, propagate carry, unshift a new leading 1 only if the whole array overflowed.

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:

Dropbox-specific tips

Dropbox interviewers like the early-return-on-no-carry trick — it mirrors how their chunk-version counter avoids re-allocating manifests when only the tail changes.

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Output

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