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7. Plus One

easyAsked at Ramp

Increment a number represented as a digit array by one.

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

Problem

You are given a large integer represented as an array of digits, where each digits[i] is the ith 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
  • The number has no leading zeros except for 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, split back into digits.

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

Tradeoff:

2. Carry propagation in place

Walk right-to-left, add carry, return early on no-carry. Handles the all-nines case by prepending one.

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:

Ramp-specific tips

Ramp graders score this on overflow-safety reasoning, which mirrors how their corporate card rules engine increments transaction counters without parsing into JavaScript's lossy Number type.

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Output

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