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#include <iostream>
#include <fstream>
#include <vector>
#include <set>
#include <queue>
#include <algorithm>
#include <string>
#include <limits>
using namespace std;
const double LARGE = numeric_limits<double>::max();
//======================================================================
struct Edge; // Forward definition
struct Vertex
{
string id;
vector<Edge *> edges;
double distance = LARGE;
Vertex *prev = nullptr;
};
struct Edge
{
Vertex *v1, *v2;
double weight;
};
struct Graph
{
vector<Vertex> vertices;
vector<Edge> edges;
Graph() {};
Graph( string filename );
void write();
};
Graph::Graph( string filename ) // Create a graph using data in file
{
int n;
int i, j;
double w;
ifstream in( filename );
in >> n;
vertices.resize( n );
for ( i = 0; i < n; i++ ) vertices[i].id = to_string( i + 1 ); // Vertex index from 0, ID from 1 (unfortunately)
while ( in >> i >> j >> w ) edges.push_back( { &vertices[i - 1], &vertices[j - 1], w } );
for ( Edge &e : edges ) e.v1->edges.push_back( &e );
}
void Graph::write()
{
cout << "Number of vertices = " << vertices.size() << '\n';
cout << "Number of edges = " << edges.size() << '\n';
cout << "Vertex list:\n";
for ( Vertex v : vertices ) cout << v.id << '\n';
cout << "Edge list:\n";
for ( Edge e : edges ) cout << e.v1->id << ' ' << e.v2->id << ' ' << e.weight << '\n';
cout << '\n';
}
//======================================================================
void printPath( const vector<Vertex *> &path )
{
cout << path[0]->id;
for ( int i = 1; i < path.size(); i++ ) cout << " -> " << path[i]->id;
cout << '\n';
}
//======================================================================
struct greaterDistance
{
bool operator() ( Vertex *a, Vertex *b ) { return a->distance > b->distance; }
};
//======================================================================
double shortestPath( Graph &g, int start, int finish, vector<Vertex *> &path )
{
Vertex *v1, *v2;
double d1, d2;
// Initialise distances and path
for ( Vertex &v : g.vertices )
{
v.distance = LARGE;
v.prev = nullptr;
}
g.vertices[start].distance = 0;
// Create a collection of pointers to vertices, sorted by distance from source
priority_queue<Vertex *, vector<Vertex *>, greaterDistance> work;
for ( Vertex &v : g.vertices ) work.push( &v );
// Loop Dijkstra's algorithm
while ( !work.empty() )
{
v1 = work.top(); // Add the current shortest-distance vertex to those finalised
work.pop(); // by removing it from the priority queue
if ( v1 == &g.vertices[finish] ) break; // Finished if we have finalised the designated node
d1 = v1->distance;
for ( Edge *e : v1->edges ) // Update any minimum distances via this node
{
d2 = d1 + e->weight;
v2 = e->v2;
if ( v2->distance > d2 )
{
v2->distance = d2;
v2->prev = v1;
work.push( v2 ); // ADDS ANOTHER COPY (with different priority)
}
}
}
// Create the path back to the source
path.clear();
path.push_back( v1 );
while ( v1->prev != nullptr )
{
v1 = v1->prev;
path.push_back( v1 );
}
reverse( path.begin(), path.end() );
return path.back()->distance;
}
//======================================================================
struct lessDistance
{
bool operator() ( Vertex *a, Vertex *b ) { return a->distance < b->distance; }
};
//======================================================================
double shortestPath_Set( Graph &g, int start, int finish, vector<Vertex *> &path )
{
Vertex *v1, *v2;
double d1, d2;
// Initialise distances and path
for ( Vertex &v : g.vertices )
{
v.distance = LARGE;
v.prev = nullptr;
}
g.vertices[start].distance = 0;
// Create a collection of pointers to vertices, sorted by distance from source
set<Vertex *, lessDistance> work;
for ( Vertex &v : g.vertices ) work.insert( &v );
// Loop Dijkstra's algorithm
while ( !work.empty() )
{
v1 = *work.begin(); // Add the current shortest-distance vertex to those finalised
work.erase( work.begin() ); // by removing it from the set
if ( v1 == &g.vertices[finish] ) break; // Get out if we have finalised the designated node
d1 = v1->distance;
for ( Edge *e : v1->edges ) // Update any minimum distances via this node
{
d2 = d1 + e->weight;
v2 = e->v2;
if ( v2->distance > d2 )
{
v2->distance = d2;
v2->prev = v1;
work.erase( v2 ); // REINSERT with new distance
work.insert( v2 ); //
}
}
}
// Create the path back to the source
path.clear();
path.push_back( v1 );
while ( v1->prev != nullptr )
{
v1 = v1->prev;
path.push_back( v1 );
}
reverse( path.begin(), path.end() );
return path.back()->distance;
}
//======================================================================
int main()
{
Graph g( "input.txt" );
g.write();
int a, b;
vector<Vertex *> path;
cout << "Input node ids for start and finish: "; cin >> a >> b;
cout << "Shortest distance (p-q): " << shortestPath ( g, a - 1, b - 1, path ) << '\n';
cout << "Path: "; printPath( path );
cout << "Shortest distance (set): " << shortestPath_Set( g, a - 1, b - 1, path ) << '\n';
cout << "Path: "; printPath( path );
}
//======================================================================
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