HDU 1548 A strange lift(Dijkstra)

HDU 1548 A strange lift(Dijkstra)

http://acm.hdu.edu.cn/showproblem.php?pid=1548

题意:有一个N层的电梯,每层有一个数字k[i],表示你在第i层能通过按up按钮到i+k[i]层去,能通过按down按钮到i-k[i]层去.(当然如果i+k[i]或i-k[i]不属于[1,N]范围,那么你不能动)现在给你层号A和B,问你通过按电梯按钮,最少几次能从A到B.

分析:

其实这就是最短路问题.把每个楼层看成有向图的一个点,当第i层能通过按钮到达j层的时候,有一条i->j的距离为1的边.现在我们只需要求从A到B的最短距离即可.

AC代码:

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<vector>
#include<queue>
#define INF 1e9
using namespace std;
const int maxn=200+10;

struct Edge
{
    int from,to,dist;
    Edge(int f,int t,int d):from(f),to(t),dist(d){}
};

struct HeapNode
{
    int d,u;
    HeapNode(int d,int u):d(d),u(u){}
    bool operator<(const HeapNode& rhs)const
    {
        return d>rhs.d;
    }
};

struct Dijkstra
{
    int n,m;
    vector<Edge> edges;
    vector<int> G[maxn];
    bool done[maxn];
    int d[maxn];

    void init(int n)
    {
        this->n=n;
        for(int i=0;i<n;i++) G[i].clear();
        edges.clear();
    }

    void AddEdge(int from,int to,int dist)
    {
        edges.push_back(Edge(from,to,dist));
        m = edges.size();
        G[from].push_back(m-1);
    }

    int dijkstra(int s,int e)
    {
        priority_queue<HeapNode> Q;
        for(int i=0;i<n;i++) d[i]=INF;
        d[s]=0;
        memset(done,0,sizeof(done));
        Q.push(HeapNode(d[s],s));
        while(!Q.empty())
        {
            HeapNode x= Q.top(); Q.pop();
            int u=x.u;
            if(done[u]) continue;
            done[u]=true;

            for(int i=0;i<G[u].size();i++)
            {
                Edge &e=edges[G[u][i]];
                if(d[e.to]>d[u]+e.dist)
                {
                    d[e.to] = d[u]+e.dist;
                    Q.push(HeapNode(d[e.to],e.to));
                }
            }
        }

        if(d[e]==INF) return -1;
        return d[e];
    }
}DJ;
int main()
{
    int n,s,e;
    while(scanf("%d",&n)==1&&n)
    {
        scanf("%d%d",&s,&e);
        s--,e--;
        DJ.init(n);

        for(int i=0;i<n;i++)
        {
            int k;
            scanf("%d",&k);
            if(i+k<n)
                DJ.AddEdge(i,i+k,1);
            if(i-k>=0)
                DJ.AddEdge(i,i-k,1);
        }

        printf("%d\n",DJ.dijkstra(s,e));

    }
    return 0;
}