poj3662 最短路+二分

  1 //Accepted    508 KB    79 ms    
  2 //spfa+二分
  3 //二分需要的花费cost，把图中大于cost的边设为1,小于cost的边设为0,然后spfa求
  4 //最短路，如果小于K则可行，继续二分 
  5 #include <cstdio>
  6 #include <cstring>
  7 #include <iostream>
  8 #include <queue>
  9 #include <cmath>
 10 #include <algorithm>
 11 using namespace std;
 12 /**
 13   * This is a documentation comment block
 14   * 如果有一天你坚持不下去了，就想想你为什么走到这儿！
 15   * @authr songt
 16   */
 17 const int imax_n = 1005;
 18 const int imax_e = 20005;
 19 const int inf = 100000000;
 20 struct node
 21 {
 22     int u,v,c;
 23     node()
 24     {
 25         
 26     }
 27     node(int u,int v,int c):u(u),v(v),c(c)
 28     {
 29         
 30     }
 31 }p[imax_e];
 32 int e;
 33 bool vis[imax_n];
 34 int head[imax_n];
 35 int next[imax_e];
 36 int dis[imax_n];
 37 int n,m,K;
 38 void init()
 39 {
 40     memset(head,-1,sizeof(head));
 41     memset(next,-1,sizeof(next));
 42     e=0;
 43 }
 44 void addEdge(int u,int v,int c)
 45 {
 46     p[e]=node(u,v,c);
 47     next[e]=head[u];
 48     head[u]=e++;
 49 }
 50 bool relax(int u,int v,int c)
 51 {
 52     if (dis[v]>dis[u]+c)
 53     {
 54         dis[v]=dis[u]+c;
 55         return true;
 56     }
 57     return false;
 58 }
 59 queue<int > q;
 60 bool spfa(int src,int len)
 61 {
 62     while (!q.empty()) q.pop();
 63     memset(vis,false,sizeof(vis));
 64     for (int i=1;i<=n;i++)
 65     {
 66         dis[i]=inf;
 67     }
 68     dis[src]=0;
 69     q.push(src);
 70     vis[src]=true;
 71     while (!q.empty())
 72     {
 73         int pre=q.front();
 74         q.pop();
 75         vis[pre]=false;
 76         for (int i=head[pre];i+1;i=next[i])
 77         {
 78             int c=p[i].c>len?1:0;
 79             if (relax(pre,p[i].v,c) && !vis[p[i].v])
 80             {
 81                 vis[p[i].v]=true;
 82                 q.push(p[i].v);
 83             }
 84         }
 85     }
 86     if (dis[n]<=K) return true;
 87     else return false;
 88 }
 89 int main()
 90 {
 91     while (scanf("%d%d%d",&n,&m,&K)!=EOF)
 92     {
 93         init();
 94         int u,v,c;
 95         int left=0,right=0,mid;
 96         for (int i=1;i<=m;i++)
 97         {
 98             scanf("%d%d%d",&u,&v,&c);
 99             right=right>c?right:c;
100             addEdge(u,v,c);
101             addEdge(v,u,c);
102         }
103         if (spfa(1,right)==0)
104         {
105             printf("-1\n");
106             continue ;
107         }
108         while (left<=right)
109         {
110             mid=(left+right)/2;
111             //printf("left=%d right=%d mid=%d\n",left,right,mid);
112             if (spfa(1,mid))
113             {
114                 right=mid-1;
115             }
116             else
117             {
118                 left=mid+1;
119             }
120         }
121         printf("%d\n",right+1);
122     }
123 }