CodeForces - 601A The Two Routes

Posted 2020-08-30 Lorazepam

http://codeforces.com/problemset/problem/601/A

这道题没想过来， 有点脑筋急转弯的感觉了

本质上就是找最短路径 但是卡在不能重复走同一个点 ---->>> 这是来坑人的

因为这是一个完全图(不是被road 连接  就是被rail连接 ) 所以一定有一条直接连1 和 n的路径

那么只用找没有连 1 和 n 的路径的 那个图的最短路即可

然后这个dijkstra写的是O(V^2)的写法 

以后尽量用优先队列的写法O(ElogV)

 1 #include <iostream>
 2 #include <stdio.h>
 3 #include <string.h>
 4 #define INF 0x3f3f3f3f
 5 using namespace std;
 6 
 7 int rail[401][401];
 8 int road[401][401];
 9 int n, m;
10 
11 int dijkstra(int town[401][401])
12 {
13     int dist[401];
14     bool use[401];
15     memset(use, 0, sizeof(use));
16     fill(dist, dist+n+1, INF);
17     dist[1] = 0;
18     while (true)
19     {
20         int v = -1;
21         for (int i = 1; i <= n; i++)
22         {
23             if (!use[i] && (v == -1 || dist[i] < dist[v])) v = i;
24         }
25         if (v == -1) break;
26         use[v] = true;
27         for (int i = 1; i <= n; i++)
28         {
29             dist[i] = min(dist[i], dist[v] + town[v][i]);
30         }
31     }
32     return dist[n];
33 
34 }
35 int main()
36 {
37     freopen("in.txt", "r", stdin);
38     scanf("%d%d", &n, &m);
39     for (int i = 1; i <= n; i++)
40     {
41         for (int j = 1; j <= n; j++)
42         {
43             rail[i][j] = INF;
44             road[i][j] = 1;
45         }
46     }
47     for (int i = 0; i < m; i++)
48     {
49         int x, y;
50         scanf("%d%d", &x, &y);
51         rail[x][y] = 1;
52         rail[y][x] = 1;
53         road[x][y] = INF;
54         road[y][x] = INF;
55     }
56     int ans = 0;
57     if (rail[1][n] == INF || rail[n][1] == INF) ans = dijkstra(rail);
58     else ans = dijkstra(road);
59     if (ans == INF) printf("-1\n");
60     else printf("%d\n",ans);
61 }