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7. Plus One

easyAsked at Roblox

Increment a large integer represented as a digit array.

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

Problem

You are given a large integer represented as an integer array digits where digits[i] is the i-th digit (most significant first). Increment the integer by one and return the resulting array of digits.

Constraints

  • 1 <= digits.length <= 100
  • 0 <= digits[i] <= 9
  • No leading zeros except for the value 0 itself

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

Parse to BigInt, add one, convert back.

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

Tradeoff:

2. Carry from right

Walk from the last digit, propagating carry. Prepend 1 if it overflows.

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;
  }
  digits.unshift(1);
  return digits;
}

Tradeoff:

Roblox-specific tips

Roblox values clean carry handling because it mirrors how virtual-currency counters tick in Robux ledgers without 64-bit overflow.

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Output

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