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8. Plus One

easyAsked at Riot Games

Increment a big-integer represented as a digit array — Riot uses this for carry-handling fluency before frame-counter arithmetic questions.

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

Problem

Given a non-empty array of digits representing a non-negative integer, increment one to the integer. The most significant digit is at the head and each element is a single digit.

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. BigInt conversion

Parse to BigInt, add 1, split back to digits.

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 tail, propagate carry, prepend 1 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;
  }
  digits.unshift(1);
  return digits;
}

Tradeoff:

Riot Games-specific tips

Riot graders prefer the explicit carry loop because game-state frame counters overflow with the same propagation pattern on a 64-Hz server tick rate.

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Output

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