poj 2983 Is the Information Reliable?

差分约束。

 

这题只需要说明是否存在环即可。距离确定的点可以用S[a] - S[b] <=len, S[b] - S[a] <= len来建。

 

然后以0点作为源点求最短路即可，判断是否有环。

 

我还是SPFA用着比较顺。。。就一直用的SPFA。。。

 

#include <cstdio>#include <cstdlib>#include <iostream>#include <string.h>#include <queue>#include <limits.h>#define MAX 1010using namespace std;typedef struct STA{int to,len;STA *next;}STA;STA *s[MAX],node[MAX*200];int n,cou;void init(){memset(s,'/0',sizeof(s));memset(node,'/0',sizeof(node));cou = 0;}void Add(int u,int v,int len){node[cou].to = v;node[cou].len = len;node[cou].next = s[u];s[u] = &node[cou++];}int SPFA(){queue<int> Q;int i,len,to,x,inq[MAX],times[MAX],dis[MAX];STA *head;memset(inq,0,sizeof(inq));memset(times,0,sizeof(times));for(i=0; i<=n; i++)dis[i] = INT_MAX;Q.push(0);inq[0] = times[0] = 1;dis[0] = 0;while( !Q.empty() ){x = Q.front();Q.pop();inq[x] = 0;head = s[x];while( head != NULL ){to = head->to;len = head->len;if( dis[to] > dis[x] + len ){dis[to] = dis[x] + len;if( !inq[to] ){Q.push(to);inq[to] = 1;times[to]++;if( times[to] > n )return -1;}}head = head->next;}}return 0;}int main(){char ch;int m,i,ans,from,to,len;while( scanf("%d%d",&n,&m) != EOF ){init();while(m--){getchar();scanf("%c",&ch);if( ch == 'P' ){scanf("%d%d%d",&from,&to,&len);Add(to,from,len);Add(from,to,-len);}if( ch == 'V' ){scanf("%d%d",&from,&to);Add(from,to,-1);}}for(i=1; i<=n; i++)Add(0,i,0);//因为下标与在左在右没啥关系，所以只需和源点联系即可。 ans = SPFA();if( ans == -1 )printf("Unreliable/n");elseprintf("Reliable/n");}return 0;}