Skip to main content

7. Plus One

easyAsked at GitLab

Increment a large integer represented as a digit array by one.

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

Problem

You are given a non-empty array digits representing a non-negative integer with most-significant digit first. Increment the number by one and return the resulting digit array.

Constraints

  • 1 <= digits.length <= 100
  • 0 <= digit <= 9
  • No leading zeros

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 convert

Join digits to BigInt, add 1, split.

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

Tradeoff:

2. Carry from right

Walk from end, propagate carry; if carry escapes, unshift 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;
  }
  digits.unshift(1);
  return digits;
}

Tradeoff:

GitLab-specific tips

At GitLab, mention how this generalizes to incrementing build-number arrays — they like seeing transfer to real release-pipeline mechanics.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Plus One and other GitLab interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →