广大暑假训练1 E题 Paid Roads(poj 3411) 解题报告

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstdlib>
 4 #include <cstring>
 5 using namespace std;
 6 
 7 const int maxn = 1000 + 5;
 8 int head[maxn];
 9 int point_vis[maxn];
10 int N, m, mincost;
11 
12 struct adjlist   // 邻接表
13 {
14     int b, c, p, r;
15     int next;
16 }node[maxn];
17 
18 void dfs(int next, int cost)
19 {
20     if (point_vis[next] > N || cost >= mincost)   // 某一点被经过的次数多于N 或者当前cost比之前求出的minsum要大就回溯
21         return;
22     if (next == N)
23     {
24         mincost = cost;
25         return;
26     }
27     for (int i = head[next]; i != -1; i = node[i].next)
28     {
29         int v = node[i].b;
30         point_vis[node[i].b]++;
31         if (point_vis[node[i].c] >= 1)    // 之前这个中间点c有访问过，就可以交纳在点c的费用p
32             dfs(node[i].b, cost+node[i].p);
33         else      // 之前该点木有走过，只能乖乖交 r 这个费用了
34             dfs(node[i].b, cost+node[i].r);
35         point_vis[node[i].b]--;    
36     }
37 }
38 
39 int main()
40 {
41     while (scanf("%d%d", &N, &m) != EOF)
42     {
43         int t1, cnt = 0;
44         memset(head, -1, sizeof(head));
45         for (int i = 0; i < m; i++)
46         {
47             scanf("%d", &t1);
48             node[cnt].next = head[t1];
49             scanf("%d%d%d%d", &node[cnt].b, &node[cnt].c, &node[cnt].p, &node[cnt].r);
50             head[t1] = cnt++;
51         }
52         memset(point_vis, 0, sizeof(point_vis));
53         mincost = maxn;
54         point_vis[1]++;
55         dfs(1, 0);
56         if (mincost == maxn)
57             printf("impossible\n");
58         else
59             printf("%d\n", mincost);
60     }
61     return 0;
62 }