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7. Plus One

easyAsked at Grab

Increment a digit-array integer by one — Grab uses this to test carry-propagation handling.

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

Problem

Given a large integer represented as an integer array digits with most significant digit first, increment the integer by one and return the resulting digit array.

Constraints

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

Convert to BigInt, add one, convert back.

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

Tradeoff:

2. Carry walk

Walk right to left adding the carry; prepend 1 if it falls off the front.

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:

Grab-specific tips

Grab interviewers like candidates who tie carry handling to fare-cent rounding in GrabPay — it shows mobile-billing intuition.

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Output

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