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7. Plus One

easyAsked at Chegg

Increment a number represented by a digit array — Chegg uses this to check carry-propagation logic for tutor-session counter rollovers.

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 ith digit of the integer. 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 except [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 round-trip

Join digits, convert to BigInt, add 1, split back.

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

Tradeoff:

2. Right-to-left carry

Walk from the last digit, propagate carries. If carry survives the loop, prepend 1.

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:

Chegg-specific tips

Chegg likes the early-return carry pattern because their session-counter increments hit the all-9s rollover only rarely and they want to see candidates optimize the common path.

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Output

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