POJ 2983 Is the Information Reliable? 差分約束

  這題用的是差分約束做的，要建立一個超級源點，P A B C 可以看做是 dist[A] - dist[B] >= c&&dist[A] - dist[B] <= c; V A B  可以看做是 dist[A] - dist[B] >= 1;

然後判斷是否符合就用spfa的性質，如果一個點進出隊列的次數大於點的個數，那麼就存在負環，就可以判定不符合.

/*
time 454MS
mem  2552K
*/
#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>
#define inf 100000005
#define maxn 1005
#define maxe 1000005
#define mem(a) memset(a, 0, sizeof(a))
using namespace std;

struct edge
{
	int v, w;
	int next;
}eg[maxe * 4];

int head[maxn], flag[maxn], flag2, egnum, num, dist[maxn], que[maxn * maxn];
bool vis[maxn];

void add(int u, int v, int w)
{
	eg[egnum].v = v;
	eg[egnum].w = w;
	eg[egnum].next = head[u];
	head[u] = egnum++;
}

void spfa()
{
	long i, u, v, w, hd = 0, tail = 0;
	mem(vis);
	memset(dist, -inf, sizeof(dist));
	que[hd++] = 0;
	dist[0] = 0;
	vis[0] = 1;
	flag[0]++;
	while (tail != hd)
	{
		u = que[tail++];
		tail = tail % (maxn * maxn);
		vis[u] = 0;
		for (i = head[u];i != -1;i = eg[i].next)
		{
			v = eg[i].v;
			w = eg[i].w;
			if (dist[v] < dist[u] + w)
			{
				dist[v] = dist[u] + w;
				if (!vis[v])
				{
					vis[v] = 1;
					que[hd++] = v;
					hd = hd % (maxn * maxn);
					flag[v]++;
					if(flag[v] > num)
					{
						flag2 = 1;
						return;
					}
				}
			}
		}
	}
	return;
}

int main()
{
	int i, a, b, c, n;
	char ch;
	while(scanf("%d%d%*c", &num, &n) != EOF)
	{
		mem(flag);
		flag2 = 0;
		egnum = 0;
		memset(head, -1, sizeof(head));
        for(i = 0;i < n;i++)
        {
        	scanf("%c", &ch);
        	if(ch == 'P')
        	{
        		scanf("%d%d%d%*c", &a, &b, &c);
	        	add(a, b, c);
	        	add(b, a, -c);
	        }
	        else
	        {
	        	scanf("%d%d%*c", &a, &b);
	            add(a, b, 1);
	        }
        }
        for(i  = 1;i <= num;i++)
        add(0, i, 0);
		spfa();
		if(flag2)
		printf("Unreliable\n");
		else
		printf("Reliable\n");
	}
	return 0;
}