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7. Plus One

easyAsked at Activision

Increment a non-negative integer represented as a digit array — Activision uses this to test edge-case discipline relevant to versioned save files.

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

Problem

Given a large integer as an array of its digits (most significant first), increment by one and return the resulting digits array.

Constraints

  • 1 <= digits.length <= 100
  • 0 <= digits[i] <= 9
  • No leading zeros except for the number 0 itself

Examples

Example 1

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

Example 2

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

Approaches

1. BigInt conversion

Join, parse as BigInt, add 1, stringify, split.

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

Tradeoff:

2. Reverse carry propagation

Walk from least-significant digit, carry while digit == 9; if we exit the loop with carry, prepend 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:

Activision-specific tips

Activision likes the carry approach because it foreshadows how their economy services handle currency overflow on cross-platform purchase aggregation.

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Output

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