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Least Missing Positive Number (Main Thread)

Here is the interview question prompt, presented for reference.

We have an unsorted array of integers such as the following:

js [0, 3, -1, -2, 1]

In the above example, the minimum number is -2 and the maximum is 3. If it were sorted, it would look like:

js [-2, -1, 0, 1, 3]

This means there is an expected range (defined as the collection of values between the minimum and maximum values) of [-2, -1, 0, 1, 2, 3] for the array.

But look at the example again:

js [-2, -1, 0, 1, 3]

We're missing a number: 2. The smallest missing positive integer for our input array is 2.

Can you write a method that takes such an array and returns the first missing positive integer?

js leastMissingPositive([1, 2, 3, 0]) // 4

In the above example, it is 4 since we already have 0 through 3. Have your code run in O(n) time with constant space (hence, we can easily determine if it was sorted, but most sorts will take O(n * log n) time).

You can see the full challenge with visuals at this link.

Challenges • Asked 9 months ago by Anonymous

Jake from AlgoDaily Commented on May 08, 2020:

This is the main discussion thread generated for Least Missing Positive Number.

Anonymous Commented on May 08, 2020:

[deleted]

Anonymous Commented on Aug 02, 2020:

this question is very ambiguous in its wording

Jake from AlgoDaily Commented on Aug 03, 2020:

Hi, happy to help. Which part is ambiguous?

Anonymous Commented on Oct 04, 2020:

Why is the return expected to be 2 in the first test case and not 0?