pb_ds的优先队列实现dijkstra

用pb_ds的优先队列来做dijkstra。。据说noip能用哟。

先来篇关于仿函数的文章。

由于pb_ds支持由迭代器访问元素，并且push操作会返回一个迭代器，merge操作会更新迭代器，相当于帮你实现了根据编号找元素的功能（每个元素对应一个迭代器）。但是由于dijkstra在取出堆顶元素以后还要知道堆顶元素的编号，于是我们可以自己搞一个结构体，然后弄一个仿函数。。这样就完成了。

 #include <cstdio>
 #include <cstring>
 #include <functional>
 #include <ext/pb_ds/priority_queue.hpp>
 using namespace std;

 const int maxn=, maxm=5e6+, INF=;

 struct heap_node{
     int value, id;
 };

 struct cmp{
     bool operator() (heap_node a, heap_node b){
         return a.value>b.value;
     }
 };

 __gnu_pbds::priority_queue<heap_node, cmp> heap;
 __gnu_pbds::priority_queue<heap_node, cmp>::point_iterator p[maxn];

 struct Edge{
     int to, next, v;
 };

 class Graph{
 private:
     int n, cntedge;
     int fir[maxn];
     Edge edge[maxm], link[maxn];
 public:
     Graph(){
         n=, cntedge=;
         memset(fir, , sizeof(fir));
         memset(edge, , sizeof(edge));
     }

     void addedge(int x, int y, int z){
         ++cntedge;
         edge[cntedge].to=y;
         edge[cntedge].v=z;
         edge[cntedge].next=fir[x];
         fir[x]=cntedge;
         return;
     }

     Edge *get_link(int x){
         int nowedge, nowson, cntlink=;
         for (nowedge=fir[x]; nowedge;
              nowedge=edge[nowedge].next){
             nowson=edge[nowedge].to;
             ++cntlink;
             link[cntlink]=edge[nowedge];
         }
         link[].v=cntlink;
         return link;
     }

 };

 int n, m, s;
 int dis[maxn];
 Graph g;

 int main(){
     for (int i=; i<maxn; ++i)
         dis[i]=INF;
     scanf("%d%d%d", &n, &m, &s);
     int f, gg, w;
     for (int i=; i<m; ++i){
         scanf("%d%d%d", &f, &gg, &w);
         g.addedge(f, gg, w);
     }
     heap_node ht;
     for (int i=; i<=n; ++i){
         ht.id=i, ht.value=INF;
         p[i]=heap.push(ht);
     }
     dis[s]=;
     ht.id=s, ht.value=;
     heap.modify(p[s], ht);
     int now, nowson;
     Edge *link;
     for (int i=; i<n; ++i){
         ht=heap.top();
         heap.pop();
         if (ht.value==INF) break;
         now=ht.id;
         link=g.get_link(now);
         for (int i=; i<=link[].v; ++i){
             nowson=link[i].to;
             if (dis[now]+link[i].v<dis[nowson]){
                 dis[nowson]=dis[now]+link[i].v;
                 ht.id=nowson, ht.value=dis[nowson];
                 heap.modify(p[nowson], ht);
             }
         }
     }
     for (int i=; i<=n; ++i){
         printf("%d ", dis[i]);
     }
     return ;
 }