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=10005, maxm=5e6+5, INF=2147483647;

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=0, cntedge=0;
        memset(fir, 0, sizeof(fir));
        memset(edge, 0, 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=0;
        for (nowedge=fir[x]; nowedge;
             nowedge=edge[nowedge].next){
            nowson=edge[nowedge].to;
            ++cntlink;
            link[cntlink]=edge[nowedge];
        }
        link[0].v=cntlink;
        return link;
    }

};

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

int main(){
    for (int i=0; i<maxn; ++i)
        dis[i]=INF;
    scanf("%d%d%d", &n, &m, &s);
    int f, gg, w;
    for (int i=0; i<m; ++i){
        scanf("%d%d%d", &f, &gg, &w);
        g.addedge(f, gg, w);
    }
    heap_node ht;
    for (int i=1; i<=n; ++i){
        ht.id=i, ht.value=INF;
        p[i]=heap.push(ht);
    }
    dis[s]=0;
    ht.id=s, ht.value=0;
    heap.modify(p[s], ht);
    int now, nowson;
    Edge *link;
    for (int i=0; i<n; ++i){
        ht=heap.top();
        heap.pop();
        if (ht.value==INF) break;
        now=ht.id;
        link=g.get_link(now);
        for (int i=1; i<=link[0].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=1; i<=n; ++i){
        printf("%d ", dis[i]);
    }
    return 0;
}