List Lunacy: Problem and Solution
I came across this interesting problem a few weeks ago, and I had a lot of fun thinking about it and solving it. In this post, I’ll cover:
 The problem statement
 Approaching the problem and coming to a solution
 Experimentally verifying the solution with an implementation
 A proof of the solution for theoretical verification
 Interesting things I found looking around after solving the problem
The Problem Statement
Write a function that returns a random node from a linked list of unknown length in O(1) space and O(n) time. (Finding a solution in O(1) time is not only algorithmically interesting, but is also realistic if the linked list is too large to fit in memory). You may only traverse through the linked list once (i.e. you can’t traverse it once to get the length of the list, then again to pick a node).
The Approach
The key thing to think about while approaching this problem is that the algorithm has to be ready to terminate at any moment because you don’t know when the list is going to end. The first thing that this tells you is that the function should always be holding onto a node that it will return if the list comes to an end. This implies that the function should pick up the first node that it comes to, and then with every node it traverses, it should replace the node it is holding with some probability. The probability of replacing the node that it’s holding with the one it’s traversing should be decreasing with the number of nodes seen because as the list gets longer, the probability of picking up a node gets smaller. The probability also depends on the number of nodes that have been traversed, but it’s not yet clear what the function is. (Think about why the probability can’t depend on something like the number of times the function replaced the node it’s holding or the number of nodes it has seen since the last replacement).
The second thing to realize will yield the solution to the problem, although initially it may not be obvious why it works. If we think about the fact that the algorithm has to be ready at any moment to terminate because it might reach the end of the list at any moment, then we realize that the algorithm always has to pick up the kth node with probabiliy 1/k. It’s clear that if the algorithm uses this method, then once the list ends, the algorithm would have picked the very last node in the list with the correct probability. If the length of the list were n, then the algorithm would pick up the last node in the list with probability 1/n. But because the algorithm has to treat every single node as if it were going to be last node in the list, then it has to do this for all the nodes in the list!
This seems unintuitive because it isn’t clear why all the other nodes also get returned with the same probability, but at the same time the reasoning appears to be correct in an odd, nonconstructivist way. Let’s write a quick implementation of this function to get some initial verification that it is correct, and then afterward we’ll do a more rigorous analysis and proof so that we can see why this solution is correct. Fortunately in this case it doesn’t take very long to write an implementation (at least in a highlevel language like Ruby), so we can do that before proving the correctness of the solution. It’ll be fun, and if we’re wrong, the output may give some clues to the correct solution. But in other cases, it may be more effective to prove that the solution is correct before diving into an implementation, because then if your code gives you the wrong answer, you can’t be entirely sure that it’s your solution and not your code that is the problem.
An Implementation
I chose to write a quick implementation in Ruby because it’d be fast to write up (and also because it’s currently my favorite language). I used an array instead of a real linked list not only because it doesn’t change the core of the algorithm and it saves time, but also because it improves clarity and avoids possible bugs in dealing with linked lists.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 

1 2 

This data yields a mean of 100,000 and a standard error of 0.2519, which gives us a good deal of confidence about the correctness of our solution. Hurray!
A Proof (no, actually two!)
So now that we have a good experimental verification that our solution is correct, let’s return back and think about why it is that our solution works. In particular, it isn’t yet clear why our method of holding onto a node at all time and replacing it with the kth node with probability 1/k results in all the nodes being picked with equal probability. Let’s take a look.I’ll go through two different proofs with two different approaches: one that immediately proves our solution correct for lists of arbitrary length, and another that proceeds using proof by induction.
Proof 1: For lists of length n
Suppose that we have a linked of length n. For the kth node in the linked list, where k = 1, 2,…,n, we pick it with probability 1/k and we hold onto our previously picked node with probability (k1)/k. In order for the kth node to be picked and held onto until the very end to be returned, the algorithm must pick node k, and then cannot pick any of the successive nodes. It doesn’t matter what the algorithm does for nodes 1 through k1 because all that matters is the last node that is picked by the algorithm.
And so, the probability that the kth node is picked and returned by the algorithm is simply the product of the probability of the kth node being picked and the probabilities that all of the successive nodes are not picked, which turns out to be:
Since this is true for all k, we see that our algorithm indeed does return a random node with equal probability. The kth node initially may be picked with probability 1/k, but with every additional node that is traversed, the probability that it is still held onto “erodes” down to 1/n.
Proof 2: A proof by induction
We proceed using proof by induction on the length of the linked list.
Basis: The linked list has length 1.
In the case of length 1, our algorithm immediately picks up the first node (with probability 1), and because the linked list ends there, it returns that node with probability 1, as desired.
Induction Step: Assume our inductive hypothesis true for all linked lists of length at most k, where k is greater than 1, and show that our inductive hypothesis holds for a linked list of length k+1.
Suppose that we have a linked list of length k+1. By the time the algorithm has traversed the first k nodes in the linked list, the node it is currently holding onto could be any of the first k nodes, each with probability 1/k, by our inductive hypothesis. When the algorithm reaches node k+1, it could pick the last node with probability 1/(k+1), or it may hold onto the previous node with probability k/(k+1), which means that if the algorithm held onto the previous node, it would be holding onto any one of the first k nodes, each with probability (1/k) * (k/(k+1)) = 1/(k+1). Hence, when the algorithm returns, it will return any of the k+1 nodes, each with probability 1/(k+1). This completes the induction step.
Additional interesting information
While writing this blog post, I was googling around, curious to see whether anyone else had writting on this problem. I ended up discovering that this problem is actually a special case of the Reservoir sampling problem, which can be described as follows: Suppose that you have a incoming stream of pieces of data. You can only see each piece of data once and you don’t know how much total data is coming through. You need to return k pieces of data from this stream, evenly distributed from the original stream.
Sounds similar, right? The algorithm for the reservoir sampling problem turns out to be quite similar to the one we’ve come up here. If you’re interested, you can read a little bit more on Wikipedia or the Dictionary of Algorithms and Data Structures, although there’s not too much information there. But there’s a very interesting post on this reservoir sampling problem and interesting extensions, such as the weighted reservoir sampling problem and the distributed reservoir sampling problem here: http://gregable.com/2007/10/reservoirsampling.html. All very interesting things to think about.