A Step-by-Step Guide with Code Walk Through

In this Leetcode problem, we’re told that we’re given an ordered array of positive integers that has some missing numbers as input, along with a number (k). Our task: find the kth missing number in that input array. Sounds more complicated than it is. Let’s get at it.

Let’s say we have an array with the number [1, 2, 3, 4], and we’re told to find the 2nd missing number. Confusing. Doesn’t seem like there is a missing number in that array.

We’d have to assume that the two missing number are 5 and 6, and thus, the 2nd missing number is 6, and the full array would actually be [1, 2, 3, 4, 5, 6].

What’s the Algorithm?

As a human, if you’re tasked to find the kth missing numbers in a sequence, I’d argue you’d do something like,

1. Start from 1, counting up
2. When a number seems to be missing, make note of it by incrementing an internal counter.
3. When you’ve reached the number of missing numbers you were asked to find, stop, and say which number should’ve been there.

Coding a Solution

Let’s initialize a function called **findKthPositive **that takes in an array (arr) and a number (k) as input. And, as described above, we’re going to need to count how many numbers are missing in our sequence, so let’s initialize a missingCounter variable:

function findKthPositive(arr, k) {
  let missingCounter = 0;
}

Next, we have to walk through our sequence. Let’s do that with a for loop. Typically we setup a for loop to run the entire length of an array. But what do we do when our array has missing values?

#leetcode #javascript