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8. Plus One

easyAsked at Coursera

Increment a digit array by one — Coursera uses this to test carry-propagation cleanliness for grade-encoded counters.

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

Problem

You are given a large integer represented as an array of digits, most significant digit first. Increment the integer by one and return the resulting digit array.

Constraints

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

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, BigInt, increment, split back. Works but discards the lesson.

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

Tradeoff:

2. Walk from end with carry

Iterate right-to-left, propagate carry, prepend 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;
  }
  digits.unshift(1);
  return digits;
}

Tradeoff:

Coursera-specific tips

Coursera looks for the in-place carry-propagation answer because they distrust BigInt branches in hot paths — explain why the in-place version is safer for grade-counter increments.

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Output

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