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7. Plus One

easyAsked at MercadoLibre

Add one to a number 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 integer by one and return the resulting array of digits.

Constraints

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

Examples

Example 1

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

Example 2

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

Approaches

1. Brute force

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. In-place carry

Walk from the right adding the carry; prepend 1 if it propagates past 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;
  }
  digits.unshift(1);
  return digits;
}

Tradeoff:

MercadoLibre-specific tips

MercadoLibre likes the carry-loop version because it mirrors how their ledger code handles overflow without falling back to BigInt allocations on hot paths.

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Output

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