问题
I just wanted to check whether there contains any duplicates in my array. I searched on Google and see some approaches:
- Double for-loop loops though the array and comparing each item
- Creating a dictionary that stores the number of occurrences of each item
But these methods require a lot of loops and I'm kind of lazy to write a large amount of code just for this functionality. xD.
So I thought of this creative way:
let containsDuplicates = Set(array).count != array.count
However, is this method faster or slower than the other two? I'm not sure because it seems to create a set which I think needs to loop through the array. And I don't know whether accessing the count also loops through the whole array.
If I only have at most 50 items in the array, will this even matter?
回答1:
That's pretty much the best way. Dictionary storage is constant time, and each item only needs to be stored once, so it's an O(n) solution, vs the O(n^2) for the double for-loop approach. The space efficiency is lower, but that's usually not a concern these days.
If you're doing this often, I would suggest using some kind of hybrid data structure, so that you're not constantly generating these Sets from scratch.
回答2:
My first thought was to create a set and to see if the number of elements was different than the array, just like you. Apple will probably have optimized this conversion fairly well and doing it this way makes the code compact and easy to understand.
You don't mention what the array is for and how it's used but could you just use a set instead of the array so that duplicates don't get added in the first place? Or if you need to keep track of how many but still need to know the unique values easily then create a new class that uses a dictionary for the storage. The key is what you are storing in the array but would be unique and the value would be the count.
来源：https://stackoverflow.com/questions/37140158/is-this-approach-to-check-duplicates-in-an-array-relatively-efficient-why-or-wh