【POJ】3169 Layout（差分约束+SPFA）

Layout

Description

Like everyone else, cows like to stand close to their friends when queuing for feed. FJ has N (2 <= N <= 1,000) cows numbered 1..N standing along a straight line waiting for feed. The cows are standing in the
same order as they are numbered, and since they can be rather pushy, it is possible that two or more cows can line up at exactly the same location (that is, if we think of each cow as being located at some coordinate on a number line, then it is possible for
two or more cows to share the same coordinate). 

Some cows like each other and want to be within a certain distance of each other in line. Some really dislike each other and want to be separated by at least a certain distance. A list of ML (1 <= ML <= 10,000) constraints describes which cows like each other
and the maximum distance by which they may be separated; a subsequent list of MD constraints (1 <= MD <= 10,000) tells which cows dislike each other and the minimum distance by which they must be separated. 

Your job is to compute, if possible, the maximum possible distance between cow 1 and cow N that satisfies the distance constraints.

Input

Line 1: Three space-separated integers: N, ML, and MD. 

Lines 2..ML+1: Each line contains three space-separated positive integers: A, B, and D, with 1 <= A < B <= N. Cows A and B must be at most D (1 <= D <= 1,000,000) apart. 

Lines ML+2..ML+MD+1: Each line contains three space-separated positive integers: A, B, and D, with 1 <= A < B <= N. Cows A and B must be at least D (1 <= D <= 1,000,000) apart.

Output

Line 1: A single integer. If no line-up is possible, output -1. If cows 1 and N can be arbitrarily far apart, output -2. Otherwise output the greatest possible distance between cows 1 and N.

Sample Input

4 2 1
1 3 10
2 4 20
2 3 3

Sample Output

27

Hint

Explanation of the sample: 

There are 4 cows. Cows #1 and #3 must be no more than 10 units apart, cows #2 and #4 must be no more than 20 units apart, and cows #2 and #3 dislike each other and must be no fewer than 3 units apart. 

The best layout, in terms of coordinates on a number line, is to put cow #1 at 0, cow #2 at 7, cow #3 at 10, and cow #4 at 27.

1、对于差分不等式，a - b <= c ，建一条 b 到 a 的权值为 c 的边，求的是最短路，得到的是最大值（本题求的就是最大值），对于不等式 a - b >= c ，建一条 b 到 a 的权值为 c 的边，求的是最长路，得到的是最小值。

2、如果检测到负环，那么无解。

3、如果d[]没有更新，那么可以是任意解。

#include<stdio.h>
#include<iostream>
#include<algorithm>
#include<string.h>
using namespace std;

const int MAXN=1010;
const int MAXE=20020;
const int INF=0x3f3f3f3f;
int head[MAXN];
int vis[MAXN];
int cnt[MAXN];
int que[MAXN];
int dist[MAXN];

struct Edge
{
int to;
int v;
int next;
}edge[MAXE];
int tol;
void add(int a,int b,int v)
{
edge[tol].to=b;
edge[tol].v=v;
edge[tol].next=head[a];
head[a]=tol++;
}
bool SPFA(int start,int n)
{
int front=0,rear=0;
for(int v=1;v<=n;v++)
{
if(v==start)
{
que[rear++]=v;
vis[v]=true;
cnt[v]=1;
dist[v]=0;
}
else
{
vis[v]=false;
cnt[v]=0;
dist[v]=INF;
}
}
while(front!=rear)
{
int u=que[front++];
vis[u]=false;
if(front>=MAXN)front=0;
for(int i=head[u];i!=-1;i=edge[i].next)
{
int v=edge[i].to;
if(dist[v]>dist[u]+edge[i].v)
{
dist[v]=dist[u]+edge[i].v;
if(!vis[v])
{
vis[v]=true;
que[rear++]=v;
if(rear>=MAXN)rear=0;
if(++cnt[v]>n) return false;
}
}
}
}
return true;
}
int main()
{
//freopen("in.txt","r",stdin);
//freopen("out.txt","w",stdout);
int n;
int ML,MD;
int a,b,c;
while(scanf("%d%d%d",&n,&ML,&MD)!=EOF)
{
tol=0;
memset(head,-1,sizeof(head));
while(ML--)
{
scanf("%d%d%d",&a,&b,&c);
if(a>b)swap(a,b);
add(a,b,c);
}
while(MD--)
{
scanf("%d%d%d",&a,&b,&c);
if(a<b)swap(a,b);
add(a,b,-c);
}
if(!SPFA(1,n)) printf("-1\n");
else if(dist
==INF) printf("-2\n");
else printf("%d\n",dist
);
}
return 0;
}

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