Are you sure you can solve this "boomerang" coding puzzle?

Are you sure you can solve this "boomerang" coding puzzle?

I found this interesting boomerang puzzle that many people could get wrong the first time.

Especially if you ask them to do it in one line — as we’re going to try to do.

const arr = [4, 9, 4, 6, 3, 8, 3, 7, 5, -5, 5];

console.log(countBoomerangs(arr)); // 3

It's all about counting the "boomerangs" in a list. Can you do it?

Maybe you first want to do what the hell a boomerang is right?

Okay so it’s like, any section of the list with 3 numbers where the first and last digits repeat: like[1, 2, 1]:

So how many boomerangs can you see in [4, 9, 4, 6, 3, 8, 3, 7, 5, -5, 5]?

…

It’s 3

1. [4, 9, 4]

2. [3, 8, 3]

3. [5, -5, 5]

So the puzzle is to write an algorithm to find this pattern throughout the list.

But here’s where it gets tricky: The algorithm should also count overlapping boomerangs — like in [1, 5, 1, 5, 1, 5, 1] we have FIVE boomerangs — not two — I even thought it was three at first — no it’s freaking five.

So how do we go about this?

My first instinct is to loop through the list and and then when we get up to 3 items we can do the calculation.

It’s one of those situations where we need to keep track of previous values in the loop at every step of the loop.

So in every loop we’ll have one value for the current item, the previous items, and the one before that.

How do we keep track of all the items?

For the current items it's super easy of course -- it's just the current iter variable:

countBoomerangs([1, 2, 1, 0, 3, 4, 3]);

function countBoomerangs(arr) {
  let curr;
  for (item of arr) {
    curr = item;
    console.log(`curr: ${curr}`);
  }
}

What about keeping tracking of the previous variable?

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Best,
The Coding Beauty team