图就是一堆点和边，但是可以用不同的方式来存储和表示。

比如这一种

#include <vector>
#include <stdlib.h>
struct Edge {
  int v; 
  int w;
  Edge* next;
  Edge():v(-1), next(NULL){}
};


struct Graph {
  int V;
  int E;
  std::vector<Edge*> adj;
  std::vector<Edge> edge_pool;
  Edge* pool_tail;

  Graph(int v, int e): V(v), E(e) {
    adj.resize(V);
    edge_pool.resize(E);
    pool_tail = &edge_pool[0];
  }
  void add_edge(int u, int v, int w) {
    pool_tail->v = v;
    pool_tail->w = w;
    pool_tail->next = adj[u];
    adj[u] = pool_tail;
    ++pool_tail;
  }
};

/*
for (Edge* e = graph->adj[i];e;e = e->next)
*/

比如这一种

#include <vector>
struct Edge {
  int src;
  int dest;
  int weight;
};
struct Graph {
  int V;
  int E;
  std::vector<Edge> edge;
  Graph(int v, int e):V(v), E(e) {
    edge.resize(E);
  }
};