POJ 1860 Currency Exchange (SPFA松弛)

 1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <queue>
 5 using namespace std;
 6 const int MAXN = 1005;
 7 struct data {
 8     int next , to;
 9     double r , c;
10 }edge[MAXN * 3];
11 int head[MAXN] , cont;
12 double d[MAXN];
13 
14 void init(int n) {
15     for(int i = 0 ; i <= n ; i++) {
16         head[i] = -1;
17         d[i] = 0;
18     }
19     cont = 0;
20 }
21 
22 inline void add(int u , int v , double r , double c) {
23     edge[cont].next = head[u];
24     edge[cont].to = v;
25     edge[cont].r = r;
26     edge[cont].c = c;
27     head[u] = cont++;
28 }
29 
30 bool spfa(int s , double V) {
31     queue <int> que;
32     while(!que.empty()) {
33         que.pop();
34     }
35     que.push(s);
36     d[s] = V;
37     while(!que.empty()) {
38         int temp = que.front();
39         que.pop();
40         for(int i = head[temp] ; ~i ; i = edge[i].next) {
41             double x = edge[i].r * (d[temp] - edge[i].c);
42             if(x > d[edge[i].to]) {  //松弛
43                 d[edge[i].to] = x;
44                 que.push(edge[i].to);
45                 if(d[s] > V) //增加则直接返回true
46                     return true;
47             }
48         }
49     }
50     return false;
51 }
52 
53 int main()
54 {
55     int n , m , s , u , v;
56     double V , r , c;
57     while(~scanf("%d %d %d %lf" , &n , &m , &s , &V)) {
58         init(n);
59         for(int i = 0 ; i < m ; i++) {
60             scanf("%d %d %lf %lf" , &u , &v , &r , &c);
61             add(u , v , r , c);
62             scanf("%lf %lf" , &r , &c);
63             add(v , u , r , c);
64         }
65         if(spfa(s , V))
66             printf("YES\n");
67         else
68             printf("NO\n");
69     }
70 }