# Find original array from array with pairs of adjacent elements

Given an array composed of pairs, like this:

[[3,5],[1,5],[3,2],[1,4]]

Each element in the array (call it pair) means that pair[0] and pair[1] are adjacent in the original array. Note, they can come in either order. For instance, for the sample array above, the original array would be:

4,1,5,3,2 (or the reversed version of this)

How can I do this quickly? I tried doing this as follows, which works, but it’s too slow:

Create a hashmap that maps adjacent elements. Map = {3: [5,2], 1: [5,4], 5: [1,3], 4: [1], 2:[3]}. My algorithm would then start with one of the keys that only has a corresponding value length of 1 (in this case either 4 or 2), and then add to an output array, and go through the hashmap. I.e. First I would add 4 to my output, and then go from key of 4 (with a corresponding value of 1), to the key of 1, with corresponding values of 5 and 4. I’d ignore 4 (since it’s already in output), and add 5, then go to the key of 5, and so on and so forth. This is too slow! Is there a better algorithm?