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8. Plus One

easyAsked at Duolingo

Add one to a non-negative 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. 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 = [9,9,9]
Output
[1,0,0,0]

Approaches

1. BigInt parse

Join digits, parse to BigInt, add 1, split back.

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

Tradeoff:

2. Walk from end with carry

Iterate right-to-left; bump the last digit; cascade the carry when a slot rolls from 9 to 0. Prepend 1 if carry remains.

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:

Duolingo-specific tips

Duolingo tracks daily streak counters as integers; the digit-carry pattern lines up with hand-rolled BCD counters used in some legacy lesson-counter modules.

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