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7. Plus One

easyAsked at Indeed

Increment a number represented as a digit array — Indeed's carry-handling warmup before pagination-counter and impression-id problems.

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

Problem

You are given a non-empty array of decimal digits representing a non-negative integer. Add one to the integer and return the resulting digit array, growing it if a carry propagates past the most significant digit.

Constraints

  • 1 <= digits.length <= 100
  • 0 <= digits[i] <= 9
  • Number has no leading zeros except 0 itself

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. BigInt convert

Convert to BigInt, add one, split back into digits.

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

Tradeoff:

2. Iterate from end

Propagate the carry from the least significant digit; prepend a 1 if it overflows.

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:

Indeed-specific tips

Indeed grades for the explicit carry-overflow case — call out the [9,9,9] -> [1,0,0,0] path since their cursor-pagination tokens have the same overflow shape.

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Output

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