Quick reference
Complexity | |
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Worst case time | |
Best case time | |
Average case time | |
Space |
Strengths:
- Fast. Merge sort runs in , which scales well as n grows.
- Parallelizable. Merge sort breaks the input into chunks, each of which can be sorted at the same time in parallel.
Weaknesses:
- Space. Merge sort takes up extra space, including space for the recursive call stack.
The High-Level Idea
Merge sort is a recursive algorithm that works like this:
- split the input in half
- sort each half by recursively using this same process
- merge the sorted halves back together
Like all recursive algorithms, merge sort needs a base case. Here, the base case is an input list with one item. A one-element list is already sorted.
Splitting the Input
Here's a visual representation of merge sort repeatedly breaking the input list in half:
At the bottom, we've got a bunch of "sorted" one-item lists that we want to combine into one large sorted list.
Merging sorted lists
Combining two one-item lists is easy—the smaller item goes first and the larger item goes second.
Now, how do we merge two of these two-item lists?
That's a bit trickier, and it definitely requires more than one comparison.
Let's come up with a more general algorithm that can merge two sorted lists of any size.
Take this example:
The smallest item has to be the first item in either the first list or the second list. So, we compare the first element in both lists and place the smaller of the two at the front of the merged list.
We can keep repeating this process, skipping over any items that we've already added to the merged list. At each step, we'll just compare the earliest unmerged items from both lists and add the smaller one.
Once we exhaust one list, we can just append any remaining items from the other one to the end of the merged list.
Using this more general approach, we can easily combine our two-element lists into four-element lists, combine those into eight-element lists, and so on, until all the items are in a single, sorted list.
Implementation
Here's how we'd code it up:
Execution Order
We described merge sort as two steps: a splitting step and a merge step.
In reality, these steps are interleaved through recursive calls.
When we split the input into two lists, we'll actually finish sorting the left half (splitting it all the way down to one-element lists and merging them back together) before we even touch the right half.
Then, we'll do the same thing with the right half before combining both sorted halves together to get the full sorted list.
Understanding the actual execution order is important for thinking through the space cost on the call stack.
Time Complexity
Look at each row in the execution diagram:
Each row in the diagram is time.
- In the upper half, we do some constant-time work for each item by copying the item from the input list into a smaller list.
- In the bottom half, we do some constant-time work for each item by copying the item from a smaller, sorted list into the larger, merged list.
How many rows are there in the diagram? In the top half, we're dividing n in half until we get down to 1. Then, in the bottom half we do the reverse—multiplying by two until we get to n. That's divisions followed by multiplications.. Adding the two together, we've got levels.
So overall, merge sort takes time.
Space Complexity
When combining two sorted lists into a single bigger one, we need scratch space to hold the merged result.
Since the lists we'll be combining have items, we'll need space in total.
But, don't we need less than space for some of the merges?
Yes. For instance, when we're merging two lists with one element each, that requires space.
But, by the time we get to the final merge, we'll have two halves that each have n/2 items, so we'll need space for the combined result.
Do we really need that scratch space? Can't we just be more clever about how we merge the sorted lists?
Try for yourself! Turns out you can't really do the merge in space without bumping up the time complexity.
If you look carefully, our implementation uses more space () than is strictly necessary.
In the arguments for each recursive call, we're passing in a copy of the input. And, since we might have recursive calls before we hit the base case, that's copies of the input.
We did this to keep the implementation nice and tidy (and not distract from the key intuition behind merge sort).
But with some cleverness, you can get it down to extra space. Try it. :)
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