Skip to main content

7. Plus One

easyAsked at Unity

Increment a large integer stored as a digit array. Unity uses this to test carry-handling on packed counters in physics state.

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

Problem

Given a non-empty array of decimal digits representing a non-negative integer, increment one to the integer. The most significant digit is first; each element contains a single digit.

Constraints

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

Examples

Example 1

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

Example 2

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

Approaches

1. Convert via BigInt

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

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

Tradeoff:

2. Right-to-left carry

Add one at the last digit, propagate carries while >=10. If a carry survives the loop, prepend a 1.

Time
O(n)
Space
O(1)
function plusOne(d) {
  for (let i=d.length-1;i>=0;i--) {
    if (d[i] < 9) { d[i]++; return d; }
    d[i] = 0;
  }
  return [1, ...d];
}

Tradeoff:

Unity-specific tips

Unity grades for branch-free arithmetic on packed counters because physics state updates run thousands of times per frame.

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 Unity interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →