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8. Plus One

easyAsked at GitHub

Increment a number represented as a digit array by one — GitHub's tiny entry to BigInt carry handling, mirroring the version-counter bump in tag-name auto-increment scripts.

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

Problem

Given a large integer represented as an integer array digits, where digits[i] is the ith digit of the integer, increment the integer by one and return the resulting digit array.

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]
Output
[1,0,0]

Approaches

1. Brute force

Convert to BigInt, add 1, split.

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

Tradeoff:

2. In-place carry from right

Walk right to left adding the carry; when a digit becomes 10 set to 0 and continue, else return. Only allocate a new array when the carry survives past the leftmost digit.

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:

GitHub-specific tips

GitHub watches for the all-9s overflow case — same edge bug that has historically bitten release-tag auto-increment scripts when major versions roll.

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Output

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