BZOJ1631 [Usaco2007 Feb]Cow Party

 1 /**************************************************************
 2     Problem: 1631
 3     User: rausen
 4     Language: C++
 5     Result: Accepted
 6     Time:196 ms
 7     Memory:2380 kb
 8 ****************************************************************/
 9  
10 #include <cstdio>
11 #include <cstring>
12 #include <algorithm>
13  
14 using namespace std;
15 const int N = 1005;
16 struct edge{
17     int next, to, v;
18 }e[100005];
19 int n, m, S, X, Y, Z, tot;
20 int q[100005], dis[N], first[N], dis1[N], ANS;
21 bool v[N];
22  
23 inline int read(){
24     int x = 0, sgn = 1;
25     char ch = getchar();
26     while (ch < ‘0‘ || ch > ‘9‘){
27         if (ch == ‘-‘) sgn = -1;
28         ch = getchar();
29     }
30     while (ch >= ‘0‘ && ch <= ‘9‘){
31         x = x * 10 + ch - ‘0‘;
32         ch = getchar();
33     }
34     return sgn * x;
35 }
36  
37 inline void add_edge(int x, int y, int z){
38     e[++tot].next = first[x], first[x] = tot;
39     e[tot].to = y, e[tot].v = z;
40 }
41  
42 void spfa(int S){
43     memset(dis, 127, sizeof(dis));
44     q[0] = S, dis[S] = 0, v[S] = 1;
45     int p, x, y;
46     for (int l = 0, r = 0; l <= r; ++l){
47         p = q[l];
48         for (x = first[p]; x; x = e[x].next){
49             y = e[x].to;
50             if (dis[p] + e[x].v < dis[y]){
51                 dis[y] = dis[p] + e[x].v;
52                 if (!v[y])
53                     v[y] = 1, q[++r] = y;
54             }
55         }
56         v[p] = 0;
57     }
58 }
59  
60 int main(){
61     n = read(), m = read(), S = read();
62     for (int i = 1; i <= m; ++i){
63         X = read(), Y = read(), Z = read();
64         add_edge(X, Y, Z);
65     }
66     for (int i = 1; i <= n; ++i){
67         spfa(i);
68         dis1[i] = dis[S];
69     }
70     spfa(S);
71     for (int i = 1; i <= n; ++i)
72         ANS = max(ANS, dis[i] + dis1[i]);
73     printf("%d\n", ANS);
74     return 0;
75 }