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8. Plus One

easyAsked at Udemy

Increment a big integer represented as a digit array — Udemy uses this to test carry handling before deeper payment-amount math problems.

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

Problem

You are given a non-negative integer represented as an array of digits, most significant digit first. Add one to it and return the result as a digit array.

Constraints

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

Join to string, convert via BigInt, add 1, split back.

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

Tradeoff:

2. In-place carry

Walk right to left, add 1 with carry. If it propagates past the front, prepend a 1.

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:

Udemy-specific tips

Udemy reviewers value candidates who avoid BigInt-by-default and reach for the in-place carry — connect it to incrementing course enrollment counts safely.

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Output

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