Understanding Time complexity calculation for Dijkstra Algorithm

问题

As per my understanding, I have calculated time complexity of Dijkstra Algorithm as big-O notation using adjacency list given below. It didn't come out as it was supposed to and that led me to understand it step by step.

1. Each vertex can be connected to (V-1) vertices, hence the number of adjacent edges to each vertex is V - 1. Let us say E represents V-1 edges connected to each vertex.
2. Finding & Updating each adjacent vertex's weight in min heap is O(log(V)) + O(1) or O(log(V)).
3. Hence from step1 and step2 above, the time complexity for updating all adjacent vertices of a vertex is E*(logV). or E*logV.
4. Hence time complexity for all V vertices is V * (E*logV) i.e O(VElogV).

But the time complexity for Dijkstra Algorithm is O(ElogV). Why?

回答1:

Dijkstra's shortest path algorithm is O(ElogV) where:

• V is the number of vertices
• E is the total number of edges

Your analysis is correct, but your symbols have different meanings! You say the algorithm is O(VElogV) where:

• V is the number of vertices
• E is the maximum number of edges attached to a single node.

Let's rename your E to N. So one analysis says O(ElogV) and another says O(VNlogV). Both are correct and in fact E = O(VN). The difference is that ElogV is a tighter estimation.

回答2:

let n be the number of vertices and m be the number of edges.

Since with Dijkstra's algorithm you have O(n) delete-mins and O(m) decrease_keys, each costing O(logn), the total run time using binary heaps will be O(log(n)(m + n)). It is totally possible to amortize the cost of decrease_key down to O(1) using Fibonacci heaps resulting in a total run time of O(nlogn+m) but in practice this is often not done since the constant factor penalties of FHs are pretty big and on random graphs the amount of decrease_keys is way lower than its respective upper bound (more in the range of O(n*log(m/n), which is way better on sparse graphs where m = O(n)). So always be aware of the fact that the total run time is both dependent on your data structures and the input class.

回答3:

In dense(or complete) graph, E logV > V^2

Using linked data & binary heap is not always best.

That case, I prefer to use just matrix data and save minimum length by row.

Just V^2 time needed.

In case, E < V / logV.

Or, max edges per vertex is less than some constant K.

Then use binary heap.

来源：https://stackoverflow.com/questions/26547816/understanding-time-complexity-calculation-for-dijkstra-algorithm