adjacency graph in c++ using stl - c++

Question

I want to make adjacency list representation of graph using vector and when we declare vector as global variable where the memory is allocated to vector in stack or heap
#include<bits/stdc++.h>
#include<vector>
using namespace std;
void addedge(vector<int> &graph, int u, int v) {
graph[u].push_back(v);
graph[v].push_back(u);
}
void printgraph(const vector<int> &gph) {
for (int v = 0 : gph) {
cout << v;
for (auto x : gph[v]) {
cout << x;
printf("\n");
}
}
}
int main() {
vector<int> gph;
addedge(gph, 2, 3);
addedge(gph, 6, 7);
addedge(gph, 1, 2);
printgraph(gph);
}

Answer 1

gph is a vector of int so you cannot access the method push_back in graph[u] because graph[u] is a int!
You can imagine an adjacency list as a space efficient matrix(2D) of int where you can have rows of different sizes.
But ultimately it is a 2D structure.
This means you have to declare your adjacency list as a vector<vector<int>>.
The following code should give you some indication on how it works:
#include<iostream>
#include<vector>
using Graph = std::vector<std::vector<int>>;
void addedge(Graph &graph, const int u, const int v) {
graph[u].push_back(v);
graph[v].push_back(u);
}
void printgraph(const Graph &gph) {
for (int node = 0 ; node < gph.size() ; node++) {
std::cout<<node<<" : ";
for (auto x : gph[node]) {
std::cout << x << " ";
}
std::cout<<std::endl;
}
}
int main() {
Graph gph(8, std::vector<int>());
addedge(gph, 2, 3);
addedge(gph, 6, 7);
addedge(gph, 1, 2);
printgraph(gph);
}

Answer 2

Rather than having an explicit parameter of your adjacency list, you could collect the data and behaviour into a class. Depending on how sparse your graph is, there are various representations available. To contrast with Davide's answer, I'll use std::multimap<int, int>.
class Graph
{
std::multimap<int, int> edges;
public:
void addedge(int u, int v)
{
edges.insert(u, v);
edges.insert(v, u);
}
friend std::ostream& operator<<(std::ostream& os, const Graph & graph)
{
for (auto v_it = graph.begin(), v_end = {}; v_it != graph.end(); v_it = v_end)
{
v_end = graph.upper_bound(v_it->first);
os << v_it->first << " : ";
for (auto it = v_it; it != v_end; ++it)
{
os << it->second << " ";
}
os << std::endl;
}
return os;
}
}
int main() {
Graph gph;
gph.addedge(2, 3);
gph.addedge(6, 7);
gph.addedge(1, 2);
std::cout << graph;
}