# Find the missing number in a sequence

Let's tackle a new coding challenge today - finding the missing number in a sequence.

Let's tackle a new coding challenge today - finding the missing number in a sequence.

Given an array of numbers from 1 to N with one number missing… Find the missing number.

Can you write a solution that solves this problem?

Go ahead and give it a try!

I’ll wait….

How did it go? Let’s chat about it.

So, one possible solution to solve this problem is by the sum of the first n natural numbers.

n*(n+1)/2.

You can find the sum of the numbers and then subtract the sum of the numbers in the array.

The result would be the missing number.

Here's the pseudocode:

1. First, find the sum of known array elements.

2. Calculate the sum of first N natural numbers using the formula n*(n+1)/2.

3. Subtract the first value from the second.

4. Voila! The result would be our missing number.

Sounds good?

Let's take a look at the JavaScript implementation:

### Time Complexity

The time complexity is O(n). This is because there is a loop that goes through each element of the array one time.

### Space Complexity

The space complexity of our solution is O(1), because it uses a constant amount of space.