Add plus one to the integer

Good morning! Here's your coding interview problem for today.

This problem was asked by Spotify.

Given a non-negative integer represented as a non-empty array of digits, plus one to the integer.

You may assume the integer do not contain any leading zero, except the number 0 itself.

The digits are stored such that the most significant digit is at the head of the list.

Example

function plusOne(digits){

};

console.log(plusOne([1,2,3])); // [1,2,4]
console.log(plusOne([4,3,2,1])); // [4,3,2,2]

Solution

function plusOne(digits) {
  let i = digits.length - 1;
  while (i >= 0 && digits[i] === 9) {
    digits[i] = 0;
    i--;
  }
  if (i === -1) {
    digits.unshift(1);
  } else {
    digits[i] += 1;
  }
  return digits;
}

console.log(plusOne([1, 2, 3])); // [1,2,4]
console.log(plusOne([4, 3, 2, 1])); // [4,3,2,2]

Implementation

• We start from the rightmost digit and keep adding one until we encounter a digit that is less than 9. This is because if the digit is 9, adding one would result in a carry-over to the next digit.
• If we encounter a digit less than 9, we simply increment it by one and return the updated array of digits.
• If we reach the leftmost digit and it was 9, we need to add a new digit at the beginning of the array before incrementing the leftmost digit. This is because adding one to 9 would result in a carry-over to the leftmost digit.