【SPFA+二分答案】BZOJ1614- [Usaco2007 Jan]Telephone Lines架设电话线

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<algorithm>
 5 #include<cmath>
 6 #include<vector>
 7 #include<queue>
 8 using namespace std;
 9 const int INF=1000000+5;
10 const int MAXN=1000+5;
11 struct edge
12 {
13     int to,dis;
14 };
15 vector<edge> E[MAXN];
16 int n,p,k;
17 int inque[MAXN],dis[MAXN];
18 
19 void addedge(int u,int v,int w)
20 {
21     E[u].push_back((edge){v,w});
22     E[v].push_back((edge){u,w});
23 }
24 
25 void init()
26 {
27     scanf("%d%d%d",&n,&p,&k);
28     for (int i=1;i<=p;i++)
29     {
30         int u,v,w;
31         scanf("%d%d%d",&u,&v,&w);
32         addedge(u,v,w);
33     }
34 }
35 
36 int check(int x)
37 {
38     queue<int> que;
39     memset(inque,0,sizeof(inque));
40     memset(dis,0x7f,sizeof(dis));
41     dis[1]=0,inque[1]=1;
42     que.push(1);
43     while (!que.empty())
44     {
45         int head=que.front();que.pop();
46         inque[head]=0;
47         for (int i=0;i<E[head].size();i++)
48         {
49             int nowlen,to=E[head][i].to;
50             if (E[head][i].dis>x) nowlen=1;else nowlen=0;
51             if (dis[head]+nowlen<dis[to])
52             {
53                 dis[to]=dis[head]+nowlen;
54                 if (!inque[to])
55                 {
56                     que.push(to);
57                     inque[to]=1;    
58                 }
59             }
60         } 
61     }
62     if (dis[n]>k) return 0;else return 1;
63 }
64 
65 void solve()
66 {
67     int lb=0,ub=INF,ans=-1;
68     while (lb<=ub)
69     {
70         int mid=(lb+ub)>>1;
71         if (check(mid)) ub=mid-1,ans=mid;
72             else lb=mid+1;
73     }
74     printf("%d\n",ans);
75 }
76 
77 int main()
78 {
79     init();
80     solve();
81     return 0;
82 }