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7. Plus One

easyAsked at Tesla

Add one to a non-negative integer represented as a digit array.

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

Problem

Given an array digits of a large non-negative integer (most-significant digit first), increment by one and return the result as an array. Handle carry through the array, possibly extending its length.

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]
Output
[1,0,0]

Approaches

1. BigInt convert

Stringify, BigInt+1, re-split.

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

Tradeoff:

2. Right-to-left carry

Walk from least-significant digit handling carry; prepend a 1 if needed.

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:

Tesla-specific tips

Tesla wants the in-place carry — avoid BigInt because embedded controllers don't have it, and the explicit loop maps cleanly to fixed-width integer math on the ECU.

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