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BZOJ1674: [Usaco2005]Part Acquisition

2024-07-18 19:32:32   222人阅读

1674: [Usaco2005]Part Acquisition

Time Limit: 5 Sec  Memory Limit: 64 MB
Submit: 259  Solved: 114
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Description

The cows have been sent on a mission through space to acquire a new milking machine for their barn. They are flying through a cluster of stars containing N (1 <= N <= 50,000) planets, each with a trading post. The cows have determined which of K (1 <= K <= 1,000) types of objects (numbered 1..K) each planet in the cluster desires, and which products they have to trade. No planet has developed currency, so they work under the barter system: all trades consist of each party trading exactly one object (presumably of different types). The cows start from Earth with a canister of high quality hay (item 1), and they desire a new milking machine (item K). Help them find the best way to make a series of trades at the planets in the cluster to get item K. If this task is impossible, output -1.

Input

* Line 1: Two space-separated integers, N and K. * Lines 2..N+1: Line i+1 contains two space-separated integers, a_i and b_i respectively, that are planet i‘s trading trading products. The planet will give item b_i in order to receive item a_i.

Output

* Line 1: One more than the minimum number of trades to get the milking machine which is item K (or -1 if the cows cannot obtain item K).

Sample Input

6 5 //6个星球,希望得到5,开始时你手中有1号货物.
1 3 //1号星球,希望得到1号货物,将给你3号货物
3 2
2 3
3 1
2 5
5 4

Sample Output

4


OUTPUT DETAILS:

The cows possess 4 objects in total: first they trade object 1 for
object 3, then object 3 for object 2, then object 2 for object 5.

HINT

 

Source

Silver

题解：
我会说我没看题就是为了写个dijkstra+heap的模版吗？
代码：
 1 #include<cstdio> 2 #include<cstdlib> 3 #include<cmath> 4 #include<cstring> 5 #include<algorithm> 6 #include<iostream> 7 #include<vector> 8 #include<map> 9 #include<set>10 #include<queue>11 #define inf 100000000012 #define maxn 50000+10013 #define maxm 200000014 #define eps 1e-1015 #define ll long long16 #define pa pair<int,int>17 using namespace std;18 inline int read()19 {20     int x=0,f=1;char ch=getchar();21     while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1;ch=getchar();}22     while(ch>=‘0‘&&ch<=‘9‘){x=10*x+ch-‘0‘;ch=getchar();}23     return x*f;24 }25 int n,m,tot;26 int d[1005],head[1005];27 bool v[1005];28 struct edge{int go,next,w;}e[50005];29 void insert(int x,int y,int z)30 {31     e[++tot].go=y;e[tot].w=z;e[tot].next=head[x];head[x]=tot;32 }33 void dijkstra()34 {35     priority_queue<pa,vector<pa>,greater<pa> >q;36     for(int i=1;i<=n;i++)d[i]=inf;37     memset(v,0,sizeof(v));38     d[1]=0;q.push(make_pair(0,1));39     while(!q.empty())40     {41         int x=q.top().second;q.pop();42         if(v[x])continue;v[x]=1;43         for(int i=head[x],y;i;i=e[i].next)44             if(d[x]+e[i].w<d[y=e[i].go])45             {46                 d[y]=d[x]+e[i].w;47                 q.push(make_pair(d[y],y));48             }49         50     }51 }52 int main()53 {54     freopen("input.txt","r",stdin);55     freopen("output.txt","w",stdout);56     m=read();n=read();57     for(int i=1;i<=m;i++)58     {59         int x=read(),y=read(),z=1;60         insert(x,y,z);61     }62     dijkstra();63     if(d[n]==inf)puts("-1");64     else printf("%d",d[n]+1);65     return 0;66 }
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