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7. Plus One

easyAsked at Electronic Arts

Increment an arbitrary-precision integer represented as a digit array.

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

Problem

Given a non-empty array of digits representing a non-negative integer (most significant digit first), add one to the integer and return the resulting 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. Convert to BigInt

Join digits, parse as BigInt, increment, then split back.

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 end; bump each 9 to 0 with a carry, otherwise increment and return.

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:

Electronic Arts-specific tips

EA favors the carry-walk approach — game-economy code at EA does atomic balance increments where overflow handling matters and BigInt allocation isn't acceptable.

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