poj 3169 差分约束（奶牛排队）

题意：1>对于ML有:d[b]-d[a]<=d；2>对于MD有:d[a]-d[b]<=-d；3>对于n个顶点的约束图有:s[i]-s[i+1]<=0

思路：差分约束。题目的解为:有负环输出-1,dis[n]无穷大输出-2,其他输出dis[n].

#include <stdio.h>
#include <string.h>
#define N 1002
#define INF 0x3fffffff
struct edge{
	int y,w,next;
}e[N*N];
int n,m1,m2,top;
int first[N],count[N],used[N],dis[N];
int q[20005];//注意队列数组的长度可能比其他的数组要长
void init(){
	int i;
	top = 0;
	memset(used,0,sizeof(used));
	memset(count,0,sizeof(count));
	memset(first,-1,sizeof(first));
	for(i = 1;i<=n;i++)
		dis[i] = INF;
}
void add(int x,int y,int w){
	e[top].y = y;
	e[top].w = w;
	e[top].next = first[x];
	first[x] = top++;
}
int relax(int x,int y,int w){
	if(dis[x] + w < dis[y]){
		dis[y] = dis[x]+w;
		return 1;
	}
	return 0;
}
int spfa(int s,int t){
	int i,front,rear,now;
	front = rear = -1;
	dis[s] = 0;
	q[++rear] = s;
	count[s]++;
	while(front < rear){
		now = q[++front];
		used[now] = 0;
		for(i = first[now];i!=-1;i=e[i].next){
			if(relax(now,e[i].y,e[i].w) && !used[e[i].y]){
				q[++rear] = e[i].y;
				used[e[i].y] = 1;
				if(++count[e[i].y] == n)//判断有无环出现
					return -1;
			}
		}
	}
	if(dis[t] == INF)
		return -2;
	return dis[t];
}
int main(){
	freopen("a.txt","r",stdin);
	while(scanf("%d %d %d",&n,&m1,&m2)!=EOF){
		int a,b,c;
		init();
		while(m1--){
			scanf("%d %d %d",&a,&b,&c);
			add(a,b,c);
		}
		while(m2--){
			scanf("%d %d %d",&a,&b,&c);
			add(b,a,-c);
		}
		printf("%d\n",spfa(1,n));
	}
	return 0;
}