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8. Plus One

easyAsked at Expedia

Increment a large integer 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 integer array digits, where each digits[i] is the ith digit of the integer. The digits are ordered from most significant to least significant. Increment the large integer by one and return the resulting array of digits.

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]
Output
[1,0]

Approaches

1. BigInt conversion

Parse, add 1, stringify, split.

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

Tradeoff:

2. Right-to-left carry

Walk from least significant digit, propagate carry. Expedia uses this approach to increment booking confirmation IDs without 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:

Expedia-specific tips

Expedia interviewers may push you to handle arbitrary-length confirmation numbers — show carry logic without overflowing fixed-width integer types.

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Output

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