1860 

  1 /*
  2 bellman_ford算法
  3 
  4 松弛操作稍微做点修改，就可判断是否存在正环了。
  5 
  6 或者执行过程中，有大于V的也可输出
  7 */
  8 // include file
  9 #include <cstdio>
 10 #include <cstdlib>
 11 #include <cstring>
 12 #include <cmath>
 13 #include <cctype>
 14 #include <ctime>
 15 
 16 #include <iostream>
 17 #include <sstream>
 18 #include <fstream>
 19 #include <iomanip>
 20 #include <bitset>
 21 #include <strstream>
 22 
 23 #include <algorithm>
 24 #include <string>
 25 #include <vector>
 26 #include <queue>
 27 #include <set>
 28 #include <list>
 29 #include <functional>
 30 
 31 using namespace std;
 32 
 33 // typedef
 34 typedef long long LL;
 35 typedef unsigned long long ULL;
 36 
 37 // 
 38 #define read freopen("in.txt","r",stdin)
 39 #define write freopen("out.txt","w",stdout)
 40 #define FORi(a,b,c) for(int i=(a);i<(b);i+=c)
 41 #define FORj(a,b,c) for(int j=(a);j<(b);j+=c)
 42 
 43 #define FF(i,a)    for(int i=0;i<(a);i+++)
 44 #define FFD(i,a)   for(int i=(a)-1;i>=0;i--)
 45 #define Z(a) (a<<1)
 46 #define Y(a) (a>>1)
 47 
 48 const double eps = 1e-6;
 49 const double INF = 1e10;
 50 const double Pi = acos(-1.0);
 51 
 52 template<class T> inline T sqr(T a){return a*a;}
 53 template<class T> inline T TMAX(T x,T y)
 54 {
 55     if(x>y) return x;
 56     return y;
 57 }
 58 template<class T> inline T TMIN(T x,T y)
 59 {
 60     if(x<y) return x;
 61     return y;
 62 }
 63 template<class T> inline void SWAP(T &x,T &y)
 64 {
 65     T t = x;
 66     x = y;
 67     y = t;
 68 }
 69 template<class T> inline T MMAX(T x,T y,T z)
 70 {
 71     return TMAX(TMAX(x,y),z);
 72 }
 73 
 74 
 75 // code begin
 76 int N,M,S;
 77 double V;
 78 
 79 #define MAXN 110
 80 struct node
 81 {
 82     double c;
 83     double r;
 84     int s;
 85     int e;
 86 }edge[MAXN*2];
 87 double dst[MAXN];
 88 
 89 bool Bellman_Ford()
 90 {
 91     // 构图完成
 92     // 用bellman-ford判断正环的存在或者最终结果比原来的大
 93 
 94     FORi(1,N+1,1) dst[i] = 0;
 95     dst[S] = V;
 96     
 97     //进行N-1次
 98     bool f;
 99     FORi(2,N+1,1)
100     {
101         // 每条边进行松弛操作
102         f = true;
103         FORj(0,2*M,1)
104         {
105             if( ( dst[ edge[j].s ] - edge[j].c )*edge[j].r>dst[ edge[j].e ]  )
106             {
107                 f = false;
108                 dst[ edge[j].e ] = ( dst[ edge[j].s ] - edge[j].c )*edge[j].r ;
109             }
110         }
111         if(f)
112         {
113             if(dst[S]>V) return true;
114             else return false;
115         }
116     }
117 
118     FORi(0,2*M,1)
119     {
120         if( (dst[edge[i].s]-edge[i].c)*edge[i].r>dst[ edge[i].e ] )
121             return true;
122     }
123     return false;
124 }
125 
126 int main()
127 {
128     read;
129     write;
130     int A,B;
131     double Rab,Cab,Rba,Cba;
132     while(scanf("%d %d %d %lf",&N,&M,&S,&V)==4)
133     {
134         FORi(0,M,1)
135         {
136             scanf("%d %d %lf %lf %lf %lf",&A,&B,&Rab,&Cab,&Rba,&Cba);
137             edge[2*i+0].s = A;
138             edge[2*i+0].e = B;
139             edge[2*i+0].r = Rab;
140             edge[2*i+0].c = Cab;
141             edge[2*i+1].s = B;
142             edge[2*i+1].e = A;
143             edge[2*i+1].r = Rba;
144             edge[2*i+1].c = Cba;
145         }
146 
147         if(Bellman_Ford()) printf("YES\n");
148         else printf("NO\n");
149         
150     }
151     return 0;
152 }
posted @ 2011-03-03 00:01  AC2012  阅读(324)  评论(0)    收藏  举报