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7. Plus One

easyAsked at Glassdoor

Increment a digit array by one — Glassdoor uses this to grade carry-handling cleanliness and how you deal with the leading-digit edge case.

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

Problem

You are given a large integer represented as an integer array digits, where each digits[i] is the i-th digit of the integer. The digits are ordered from most significant to least. Increment the large integer by one and return the resulting array of digits.

Constraints

  • 1 <= digits.length <= 100
  • 0 <= digits[i] <= 9
  • digits has no leading zeros except 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 to number

Parse to BigInt, add one, stringify.

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

Tradeoff:

2. Right-to-left carry

Walk from the end, 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:

Glassdoor-specific tips

Glassdoor likes candidates who reject the BigInt trick and reach for carry-propagation — their numeric review aggregators run on bounded buffers and similar arithmetic instincts.

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Output

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