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8. Plus One

easyAsked at Brex

Increment an integer represented as a digit array by one and return the resulting digits.

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

Problem

Given a non-empty array of decimal digits representing a non-negative integer (most significant digit first), increment one to the integer. Return the resulting array of digits.

Constraints

  • 1 <= digits.length <= 100
  • 0 <= digits[i] <= 9
  • 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. BigInt conversion

Convert to BigInt, add 1, split digits back. Works but skips the carry pattern.

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

Tradeoff:

2. In-place carry from end

Walk from the last digit; if 9 set to 0 and continue carry, else +1 and return. Prepend 1 if loop completes.

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:

Brex-specific tips

Brex graders use this to probe how you handle integer-precision in ledger systems, so volunteer that production code should use a decimal/cents type, never floating point, for currency.

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Output

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