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7. Plus One

easyAsked at Monzo

Add one to a number represented as a digit array (think transaction counter).

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. Increment the large integer by one and return the resulting array of digits.

Constraints

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

Examples

Example 1

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

Example 2

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

Approaches

1. Parse to number

Convert to BigInt, add 1, convert back. Fragile and costly.

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

Tradeoff:

2. Carry walk

Walk right-to-left propagating carry; prepend 1 only if needed.

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:

Monzo-specific tips

Monzo doesn't want BigInt tricks where a carry walk works — they look for primitives, not language gymnastics.

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Output

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