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7. Plus One

easyAsked at Lyft

Add one to a number represented as a digit array.

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

Problem

Given a non-empty array of decimal digits representing a non-negative integer, increment the number by one. The most significant digit is first and each element contains a single digit.

Constraints

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

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

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

Tradeoff:

2. Carry from the right

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

Tradeoff:

Lyft-specific tips

Lyft asks about integer overflow even on warm-ups — explain why the carry approach avoids any 64-bit limits when ride counters grow huge.

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Output

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