Skip to main content

8. Plus One

easyAsked at SoFi

Increment a number stored as a digit array — SoFi uses this as a warmup for big-integer arithmetic that appears in their interest-precision code.

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

Problem

You are given a large integer represented as an array of digits, most significant first. Increment the integer by one and return the resulting array.

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. Brute force

Convert to BigInt, add 1, convert back.

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

Tradeoff:

2. Carry propagation

Walk from least significant digit, propagating the carry. SoFi prefers this since it avoids BigInt and demonstrates you understand financial-precision arithmetic.

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:

SoFi-specific tips

SoFi prefers the carry-propagation answer because their interest-accrual code never uses floats; demonstrate you understand why.

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

Practice these live with InterviewChamp.AI →