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8. Plus One

easyAsked at Gojek

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

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 i-th digit of the integer in left-to-right order. 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 = [9,9,9]
Output
[1,0,0,0]

Approaches

1. Convert to BigInt

Join digits, BigInt add, split back. Works but allocates strings.

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, propagate a carry, and prepend a 1 if it survives the leftmost slot.

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:

Gojek-specific tips

Gojek processes huge volumes of low-value transactions across SEA, so showing care for fixed-precision arithmetic without resorting to BigInt earns bonus credit.

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Output

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