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7. Plus One

easyAsked at Booking

Increment a digit-array by one — Booking uses this to baseline carry-handling logic seen in pricing-rule cascades.

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

Problem

Given a large integer represented as an integer array digits, increment the 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 = [9,9]
Output
[1,0,0]

Approaches

1. BigInt conversion

Join digits, BigInt-add, split — works but burns memory.

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 last digit, add one, propagate carry; if carry remains, prepend 1.

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:

Booking-specific tips

Booking grades for handling carry-style cascades cleanly — connect this to incrementing a count-of-nights rate without breaking precision in a pricing engine.

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Output

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