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7. Plus One

easyAsked at Zoom

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

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

Problem

Given a non-empty array of decimal digits representing a non-negative integer, increment by one. The most significant digit is at the head of the list. Each element is a single digit; the integer has no leading zeros.

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]
Output
[1,0,0]

Approaches

1. BigInt convert

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. Right-to-left carry

Walk from the end, propagating the carry until it stops. Prepend 1 only if we overflow.

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:

Zoom-specific tips

Zoom uses digit-carry problems as a proxy for sequence-counter overflow logic in their RTP packet ordering layer — bring up overflow semantics.

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Output

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