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## June 30, 2017 Generating Permutations

Continuing on last week’s theme, this week I’d like to share my favorite methods for generating permutations of elements!
Practically speaking, we encounter permutations less often so why should we spend time on it? Well because it is a fundamental problem in computing, it provides a basis for backtracking algorithms, and we can use it for computing exact answers to some problems. Also there are dozens of algorithms which are both fascinating and fun!
Right now we will take a look at a few key algorithms and, by the end, we will have a good grasp of what’s out there and how to think about permutation algorithms. The last algorithm I will describe will be my favorite – simple, elegant, and efficient to use.

## Brief Recap

A “permutation”, as we may remember from high school, is an re-ordering of elements. For example these are all the permutations of three elements:

    {1,2,3}
{1,3,2}
{2,1,3}
{2,3,1}
{3,1,2}
{3,2,1}


Basically we pick the first element from the n items, the second from the remaining (n-1) items, the third from the remaining (n-2) items and so on. This gives us a total of
n × (n-1) × (n-2)... × 2 × 1 items. This is, of course, the definition of n!.

## Basic Algorithm 1: Remove

Given we know there are n! permutations of elements we are lead directly to a basic backtracking algorithm for permutations –

• Remove each element from the n elements one at a time, then append it to the (n-1)! remaining permutations.

This is pretty much a direct definition of n!=n × (n-1)! and is very simple to implement:

    public void permutation1(int[] a, int n) {
if(n == 1) {
System.out.println(Arrays.toString(a));
return;
}
for(int i = 0;i < n;i++) {
swap(a, i, n-1);  // (remove the ith element)
permutation1(a, n-1);
swap(a, i, n-1);  // (restore for the next round)
}
}


How efficient is this algorithm? Well it is simple and does O(n!) as expected but it does two swaps for each pass. Because n! is large and swaps are expensive, we should try to do better.

### Side Note: Decrease and Conquer

This is an example of the “decrease and conquer” algorithmic strategy. On each pass through the loop, we peel off a value, solve the rest of the problem, and then make a change.
This “decrease and conquer” is typical and is known as decrease-by-one. It has the following characteristics:

1. It divides the problem into two parts: a sub-problem of size (n -1) and a single remaining element.
2. It then solves the sub-problem of size (n-1) by a recursive call (or an iterative decreasing process).
3. Finally, it adds the remaining individual element back into the sub-problem’s solution.

Even more so than in divide-and-conquer, the ‘divide’ step is often trivial. Most of the work goes into the third step, incorporating the lone element into the existing sub-solution.

## Basic Algorithm 2: Insert

Now let us try again. There is another very simple bottom up decomposition of n! that is the “opposite” of our first attempt:

• Insert the nth element at all possible locations of the (n-1)! remaining permutations.

Let’s look at an example:

                        1
21                     12
321 231 213        312 132 123


This is also quite simple to implement:

    public ArrayList<ArrayList<Integer>> permutation2(int[] a, int n) {
ArrayList<ArrayList<Integer>> gen = new ArrayList<>();
if(n == 1) {
ArrayList<Integer> new_permutation = new ArrayList<>();
} else {
Iterator<ArrayList<Integer>> itr = permutation2(a, n-1).iterator();
while(itr.hasNext()) {
ArrayList<Integer> permutation = itr.next();
// (create new permutation with this element in every position)
for(int i = 0;i <= permutation.size();i++) {
ArrayList<Integer> new_permutation = new ArrayList<>(permutation);
}
}
}
return gen;
}


How efficient is this minimal-change algorithm? Time-wise we can’t do much better but we are generating and storing all the permutations from (n-1), (n-2), ..., down to 1. That’s expensive so let’s try again.

### Side Note: Minimal Change

The algorithm above works but the output can be improved. Notice that many times we are simply exchanging consecutive numbers – but not for the step between 213 and 312. This means that we can’t count on our algorithm taking constant time per generation which could be important. Adding a small tweak, we can fix this:

• When inserting the nth element for each of the remaining (n-1)! permutations, start from the right and move left, then start from the left and move right.
                        1
21                    12
321 231 213       123 132 312


This will result in all steps being just swaps between adjacent elements. Algorithms which generate permutations by adjacent swaps are known as “minimal change” algorithms.

## Basic Permutation 3: Lexicographic

Here is another idea. What if we generated permutations just by taking the existing permutation and modifying it slightly? This seems like a nice idea too but brings up a couple of difficulties:

• How would we not repeat permutations without keeping track of all permutations generated so far? We could take in permutation {...xy...}, modify it to {…yx…}, and in the very next step modify it back to {…xy…}!
• And how would we know when to stop?

Both problems can be solved by defining an ordering of permutations. Once we do that we can just start with the “smallest” permutation and increment it minimally till we reach the “largest” permutation.
This is easy to do with our examples so far – we’ve used digits and so we can think of each permutation as numbers with all the ordering goodness that comes from that. What can we do when we are given other elements? Well we can simply transform them into numbers as “indexes” into the elements given (like array indices).
Let us look at permutations as numbers:

    1234
1243
1324
1342
1423
1432
2134
...


In the example above we see that 1 stays as the first number for a long time as there are many reorderings of the last 3 digits which increase the permutation by a smaller amount.
So when do we finally “use” the 1? When there are only no more permutations of the last 3 digits. And when are there no more permutations of the last 3 digits? When the last 3 digits are in descending order.
Hence we only change the position of a “digit” when everything to the right is in descending order. Therefore we start with all digits in ascending order and permute until all they reverse direction.
How exactly is this done? When everything to the right of a digit is in descending order, we find the next largest digit and put it in front and then put the remaining digits back in ascending order. This gives us the lexicographic permutation algorithm that is used in the GNU C++ std::next_permutation

## Steinhaus–Johnson–Trotter algorithm

This is the most well-known historically of the permutation algorithms. It is efficient and useful as well and we now know enough to understand it pretty easily.
The algorithm derives from “Basic Permutation 2: Insert” and is, in essence, the same as the “minimal change” version we saw earlier. It adds lexicographic ordering to figure out how to generate permutations and change direction. We can understand how it work as follows:

• Put the nth element in all positions. This, if we look at it in action, makes it look like it is “moving” from one end to the other
• 1 2 3 < 4
• 1 2 < 4 3
• 1 < 4 2 3
• < 4 1 2 3
• Now generate the next permutation of the remaining (n-1)! elements by using the same logic (i.e. starting to “move” the next highest element)
• <4 1 < 3 2
• Now that we have the next permutation, move the nth element again – this time in the opposite direction (exactly as we wanted in the “minimal changes” section)
• 1 4 >< 3 2
• 1 < 3 4 > 2
• 1 < 3 2 4 >
• Repeat this until nothing can move.

The algorithm itself follows:

• Set a direction for each position to move in
• < 1 < 2 < 3 < 4
• An element can move if it is larger than the element it is moving to
• < 1 < 2 < 3 < 4 (all of these can move)
• < 4 < 3 < 2 < 1 (none of these can move)
• Move the largest element that can move
• Change the direction of all elements that are bigger than the moved element
    public void sjt_algorithm(int[] a, int n) {
int moving;
// (initialize direction)
int[] dirs = new int[a.length];
for(int i = 0;i < dirs.length;i++) dirs[i] = -1;
int x = 1;
System.out.println(Arrays.toString(a));
while((moving = get_largest_moveable(a, dirs, n)) != -1) {
// (reverse direction of all larger numbers)
for(int i = 0;i < dirs.length;i++) {
if(a[i] > a[moving]) dirs[i] = dirs[i] * -1;
}
// (move the current largest)
move(a, dirs, moving);
System.out.println(Arrays.toString(a));
}
}


## Heap’s Algorithm

Finally we come to my favorite algorithm. It is small, efficient, and elegant and brilliantly simple in concept. We use the first and simplest concept we came up with “Basic Permutation 1: Remove” i.e. remove each element in turn and recursively generate the remaining permutations.
The problem we faced in a naive implementation was we had to do two swaps in order to pick the next element to remove.
Now consider this – what if we had some clever way to keep track of which elements we had already removed? Then we could just swap out unremoved elements and never need to do the second swap to restore order!
This is basically what Heap found – a method for picking the element to swap so that it is different in each case. The way it works is:

• If the number of elements is odd, always pick the first one
• If the number of elements is even, pick them up sequentially

Sadly I have never been able to develop a complete intuition for why this works so I just memorize it.
Because it is hard to develop an intuition for Heap’s Algorithm there are several incorrect implementations floating around the net. In fact, the Wikipedia page for Heap’s algorithm had a bug until 2015, and had to be fixed again twice in 2016 because it was edited incorrectly. This is the correct version:

    public void heaps_algorithm(int[] a, int n) {
if(n == 1) {
// (got a new permutation)
System.out.println(Arrays.toString(a));
return;
}
for(int i = 0;i > (n - 1);i++) {
heaps_algorithm(a, n-1);
// always swap the first when odd,
// swap the i-th when even
if(n % 2 == 0) swap(a, n-1, i);
else swap(a, n-1, 0);
}
heaps_algorithm(a, n-1);
}


As you can see, it is small and neat. The only tricky bit you need to keep in mind is that the loop index goes from 0 to (n-1) which means we need an extra recursive call outside the loop.
I hope you have enjoyed this tour and now feel that generating permutations is a fascinating topic with lots of interesting algorithms. Which methods did you like? Let me know in your comments below.

charles.lobo

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