Easiest way of using min priority queue with key update in C++

Question

Sometimes during programming contests etc., we need a simple working implementation of min priority queue with decrease-key to implement Dijkstra algorithm etc.. I often use

Answer 1

Actually, there is a way to use built-in binary heap from the standard c++ library. The key observation is that the implementation of all heap functions (i.e. std::push_heap, std::pop_heap and std::make_heap) need only the following methods from the stored element of class A:

• Constructor A::A()
• Assignment operator A& A::operator=(const A& rhs)
• Comparison operator bool operator<(const A& lhs, const A& rhs)

It means that assignment operator is called whenever an element is moved in the container storing all heap elements. By overloading this operator you can control the index in the heap. When you have the index you can access the element in the heap, change its value and call std::push_heap to update its position in the heap.

See the simplified implementation of Dijkstra algorithm (without graph):

#include <bits/stdc++.h>

using namespace std;

vector<int> queue_idx;

struct Elem {
    int label;
    int dist;
    bool operator<(const Elem& other) const { return dist > other.dist; }
    Elem& operator=(const Elem& other);
};

vector<Elem> q;

Elem& Elem::operator=(const Elem& other)
{
    label = other.label;
    dist = other.dist;
    queue_idx[label] = this - q.data();
    return *this;
}

void AddElem(int label, int dist)
{
    queue_idx[label] = label;
    q.push_back(Elem{label, dist});
}

void RemoveMin()
{
    pop_heap(q.begin(), q.end());
    Elem res = q.back();
    q.pop_back();
    cout << "dist to " << res.label << " is " << res.dist << endl;
}

void Relax(int label, int dist)
{
    int idx = queue_idx[label];
    Elem& elem = q[idx];
    if (elem.dist > dist)
    {
        elem.dist = dist;
        push_heap(q.begin(), q.begin() + idx + 1);
    }
}

int main()
{
    int n = 5;
    queue_idx.resize(n);
    AddElem(0, 0);
    for (int i = 1; i < n; ++i)
        AddElem(i, INT_MAX);
    make_heap(q.begin(), q.end());
    RemoveMin();
    Relax(1, 50);
    Relax(2, 40);
    Relax(4, 10);
    RemoveMin();
    Relax(3, 20);
    RemoveMin();
    Relax(1, 30);
    RemoveMin();
    Relax(2, 80);
    RemoveMin();
    return 0;
}

I know that this solution depends on the inner implementation of the standard library, however it just works for any compiler I am aware of and I used it in programming competitions.

Answer 2

I don't think the std::priority_queue class allows for an efficient implementation of decrease-key style operations.

I rolled my own binary heap based data structure that supports this, bascially along very similar lines to what you've described for the std::set based priority queue you have:

• Maintain a binary heap, sorted by value that stores elements of pair<value, ID> and an array that maps ID -> heap_index. Within the heap routines heapify_up, heapify_down etc it's necessary to ensure that the mapping array is kept in-sync with the current heap position of elements. This adds some extra O(1) overhead.
• Conversion of an array to a heap can be done in O(N) according to the standard algorithm described here.
• Peeking at the root element is O(1).
• Checking if an ID is currently in the heap just requires an O(1) look-up in the mapping array. This also allows O(1) peeking at the element corresponding to any ID.
• Decrease-key requires an O(1) look-up in the mapping array followed by an O(log(N)) update to the heap via heapify_up, heapify_down.
• Pushing a new item onto the heap is O(log(N)) as is popping an exitsing item from the heap.

So asymptotically the runtime is improved for a few of the operations compared with the std::set based data structure. Another important improvment is that binary heaps can be implemented on an array, while binary trees are node-based containers. The extra data locality of the binary heap usually translates to improved runtime.

A few issues with this implementation are:

• It only allows integer item ID's.
• It assumes a tight distribution of item ID's, starting at zero (otherwise the space complexity of the mapping array blows up!).

You could potentially overcome these issues if you maintained a mapping hash-table, rather than a mapping array, but with a little more runtime overhead. For my use, integer ID's have always been enough.

Hope this helps.

Answer 3

Although my response will not answer the original question, I think it could be useful for people who reach this question when trying to implement Dijkstra algorithm in C++/Java (like myself), something that was comment by the OP,

priority_queue in C++ (or PriorityQueue in Java) do not provide a decrease-key operation, as said previously. A nice trick for using those classes when implementing Dijkstra is using "lazy deletion". The main loop of Dijkstra algorithm extracts the next node to be processed from the priority queue, and analises all its adjacent nodes, eventually changing the cost of the minimal path for a node in the priority queue. This is the point where decrease-key is usually needed in order to update the value of that node.

The trick is not change it at all. Instead, a "new copy" for that node (with its new better cost) is added into the priority queue. Having a lower cost, that new copy of the node will be extracted before the original copy in the queue, so it will be processed earlier.

The problem with this "lazy deletion" is that the second copy of the node, with the higher bad cost, will be eventually extracted from the priority queue. But that will be always occur after the second added copy, with a better cost, has being processed. So the very first thing that the main Dijkstra loop must do when extracting the next node from the priority queue is checking if the node has being previously visited (and we know the shortest path already). It is in that precise moment when we will be doing the "lazy deletion" and the element must be simply ignored.

This solution will have a cost both in memory and time, because the priority queue is storing "dead elements" that we have not removed. But the real cost will be quite small, and programming this solution is, IMHO, easier than any other alternative that tries to simulate the missing decrease-key operation.

Answer 4

Well, as Darren already said, std::priority_queue doesn't have means for decreasing the priority of an element and neither the removal of an element other than the current min. But the default std::priority_queue is nothing more than a simple container adaptor around a std::vector that uses the std heap functions from <algorithm> (std::push_heap, std::pop_heap and std::make_heap). So for Dijkstra (where you need priority update) I usually just do this myself and use a simple std::vector.

A push is then just the O(log N) operation

vec.push_back(item);
std::push_heap(vec.begin(), vec.end());

Of course for constructing a queue anew from N elements, we don't push them all using this O(log N) operation (making the whole thing O(Nlog N)) but just put them all into the vector followed by a simple O(N)

std::make_heap(vec.begin(), vec.end());

The min element is a simple O(1)

vec.front();

A pop is the simple O(log N) sequence

std::pop_heap(vec.begin(), vec.end());
vec.pop_back();

So far this is just what std::priority_queue usually does under the hood. Now to change an item's priority we just need to change it (however it may be incorporated in the item's type) and make the sequence a valid heap again

std::make_heap(vec.begin(), vec.end());

I know this is an O(N) operation, but on the other hand this removes any need for keeping track of an item's position in the heap with an additional data structure or (even worse) an augmentation of the items' type. And the performance penalty over a logarithmic priority update is in practice not that signficant, considering the ease of use, compact and linear memory useage of std::vector (which impacts runtime, too), and the fact that I often work with graphs that have rather few edges (linear in the vertex count) anyway.

It may not in all cases be the fastest way, but certainly the easiest.

EDIT: Oh, and since the standard library uses max-heaps, you need to use an equivalent to > for comparing priorities (however you get them from the items), instead of the default < operator.