pku 2983 Is the Information Reliable?(差分约束系统)

差分约束系统

构图

A=B+x  =>  A-B>=x && A-B<=X

 

A>=B+1

判断负环

因为只需要判断是否存在负环，所以无需添加超级源点。直接把所有节点的dist属性初始化为0。

#include <iostream>using namespace std;struct Edge{int x,y,dist;}edges[200005];int edge_num,N,M;int dist_array[1005];void AddEdge(int x,int y,int dist){edges[edge_num].x=x;edges[edge_num].y=y;edges[edge_num].dist=dist;edge_num++;}bool Bellman_Ford(){bool change;for(int i=0;i<N;i++){change=false;for(int j=0;j<edge_num;j++){if(dist_array[edges[j].x]+edges[j].dist<dist_array[edges[j].y]){dist_array[edges[j].y]=dist_array[edges[j].x]+edges[j].dist;change=true;}}}return !change;}int main(){while(scanf("%d%d",&N,&M)!=EOF){memset(dist_array,0,sizeof(dist_array));edge_num=0;int x,y,dist;char flag;for(int i=0;i<M;i++){scanf("/n%c%d%d",&flag,&x,&y);if(flag=='P'){scanf("%d",&dist);AddEdge(x,y,-dist);AddEdge(y,x,dist);}else{AddEdge(x,y,-1);}}if(Bellman_Ford())printf("Reliable/n");else printf("Unreliable/n");}return 0;}