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review kruskal: some refactoring + todos

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This commit is contained in:
Maximilian Keßler 2024-08-02 00:32:04 +02:00
parent 10ae6db373
commit 79c7bfeb0d
Signed by: max
GPG key ID: BCC5A619923C0BA5
2 changed files with 44 additions and 42 deletions
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51
min_spanning_tree/kruskal/kruskal.cpp
View file

@ -2,16 +2,12 @@
// Authors: Georǵi Kocharyan
#include <iostream>
#include <cstdio>
#include <vector>
#include <list>
#include <queue>
#include <algorithm>
#include <string>
#include <functional>
#include "../../weighted_graph.h"
#include <tuple>
#include "../../weighted_graph.h"
struct key_compare
{
@ -21,9 +17,18 @@
}
};
void next_edge(WeightedGraph const & G, std::vector<std::list<int>> & elements, std::vector<int> component) {
struct Edge
{
// todo: use type alias
int from;
int to;
double weight;
bool operator<(Edge const & other) const {
return weight < other.weight;
}
};
void kruskal(WeightedGraph const & G) {
// preprocessing: remove all double edges except the minimal ones
@ -33,7 +38,7 @@ void kruskal(WeightedGraph const & G) {
// preprocessing: create a vector of lists tracking the elements of the components
std::vector<std::list<int>> elements;
std::vector<std::vector<int>> elements;
elements.resize(H.num_nodes());
for (int i = 0; i < H.num_nodes(); i++)
{
@ -52,15 +57,14 @@ void kruskal(WeightedGraph const & G) {
// preprocessing: create a vector containing all edges in O(m)
// then sort them according to their weight
// todo: use std::vector<Edge>
std::vector<std::tuple<int, int, double>> edges;
edges.resize(H.num_edges());
int count = 0;
edges.reserve(H.num_edges());
for (int i = 0; i < H.num_nodes(); i++)
{
for (const auto& j : H.adjList(i))
{
edges[count] = std::make_tuple(i,j.first,j.second);
count++;
edges.emplace_back(i,j.first,j.second);
}
}
std::sort(edges.begin(), edges.end(), key_compare());
@ -68,30 +72,28 @@ void kruskal(WeightedGraph const & G) {
for (const auto& edge : edges)
{
// check if edge connects two of the same component
if (!(component[std::get<0>(edge)]==component[std::get<1>(edge)]))
if (component[std::get<0>(edge)] != component[std::get<1>(edge)])
{
// output edge
std::cout << std::get<0>(edge) << "-" << std::get<1>(edge) << "\t" << std::get<2>(edge) << std::endl;
total_weight = total_weight + std::get<2>(edge);
total_weight += std::get<2>(edge);
// make the components of each node the same
// the larger component absorbs the second to guarantee O(mlogn) runtime
// todo: avoid code duplication by moving out stuff from if-else
int moved;
int movedto;
if (elements[component[std::get<0>(edge)]].size() >= elements[component[std::get<1>(edge)]].size())
{
// move all elements of second component to first
int moved = component[std::get<1>(edge)];
int movedto = component[std::get<0>(edge)];
for (const auto& node : (elements[moved]))
{
elements[movedto].push_back(node);
component[node] = movedto;
}
elements[moved].clear();
moved = component[std::get<1>(edge)];
movedto = component[std::get<0>(edge)];
}
else
{
// move all elements of first component to second
int moved = component[std::get<0>(edge)];
int movedto = component[std::get<1>(edge)];
moved = component[std::get<0>(edge)];
movedto = component[std::get<1>(edge)];
}
for (const auto& node : (elements[moved]))
{
elements[movedto].push_back(node);
@ -99,7 +101,6 @@ void kruskal(WeightedGraph const & G) {
}
elements[moved].clear();
}
}
}
std::cout << "The total weight of the MST is " << total_weight << std::endl;

19
weighted_graph.h
View file

@ -7,23 +7,24 @@
struct Node {
// todo: introduce neighbor struct and use it instead of std::pair
std::list<std::pair<int, double>> neighbours;
Node() = default;
Node(std::list<std::pair<int, double>> neighbours);
explicit Node(std::list<std::pair<int, double>> neighbours);
void add_edge(int node_id, double weight);
int deg() const;
[[nodiscard]] int deg() const;
};
class WeightedGraph {
public:
WeightedGraph(size_t num_nodes);
explicit WeightedGraph(size_t num_nodes);
void add_edge(int from, int to, double weight);
std::list<std::pair<int, double>> adjList(int node_id) const;
size_t num_nodes() const;
int num_edges() const;
int deg(int v) const;
std::pair<int,double> min_neighbour(int node_id) const;
WeightedGraph remove_parallel() const;
[[nodiscard]] std::list<std::pair<int, double>> adjList(int node_id) const;
[[nodiscard]] size_t num_nodes() const;
[[nodiscard]] int num_edges() const;
[[nodiscard]] int deg(int v) const;
[[nodiscard]] std::pair<int,double> min_neighbour(int node_id) const;
[[nodiscard]] WeightedGraph remove_parallel() const;
private:
std::vector<Node> nodes;