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7. Plus One

easyAsked at TripAdvisor

Increment a large integer represented as an array of digits.

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 significant. 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 = [9,9]
Output
[1,0,0]

Approaches

1. Bigint parse

Join, BigInt add, split back to digits.

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

Tradeoff:

2. Right-to-left carry

Walk from least significant, propagate carry. Prepend 1 if final carry.

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:

TripAdvisor-specific tips

TripAdvisor uses incremental-counter problems to gauge if you handle overflow cleanly when bumping review counters and trip-itinerary versions.

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Output

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