1993 小k的农场

1993 小k的农场

差分约束的题一般都是用SPFA进行解决
输入多种不同的路径，差分路径，权值路劲，双向路径
然后判断是否满足最短路性质

#include<iostream>
#include<algorithm>
#include<string>
#include<cstring>
#include<cstdio>
#include<queue> 
using namespace std;
const int SIZE=5005;
int n, m, cnt, elast[SIZE], dis[SIZE], num[SIZE];
bool vis[SIZE];
struct edge
 {
	int to, len, next;
} e[15005];
queue<int> q;
void add (int u, int v, int w) //邻接表 
{
	e[++cnt].to = v;
	e[cnt].len = w;
	e[cnt].next = elast[u];
	elast[u] = cnt;
}

bool spfa (int x) 
{
	dis[x] = 0;
	q.push(x);
	vis[x] = true;
	num[x]++;//计算答案 
	while (!q.empty()) 
	{
		int u = q.front();
		q.pop();
		vis[u] = false;
		for (int i = elast[u]; i != 0; i = e[i].next)
			if (dis[e[i].to] > dis[u] + e[i].len) 
			{
				dis[e[i].to] = dis[u] + e[i].len;
				if (!vis[e[i].to]) 
				{
					q.push(e[i].to);
					vis[e[i].to] = true;
					num[e[i].to]++;//计算答案 
					if (num[e[i].to] == n + 1)//无解 
						return false;
				}
			}
	}
	return true;
}

int main () 
{
	scanf("%d %d", &n, &m);
	memset(dis, 0x3f3f3f3f, sizeof(dis));
	for (int i = 1; i <= m; i++) 
	{
		int opt;
		scanf("%d", &opt);
		switch (opt) 
		{
			case 1: 
			{
				int a, b, c;
				scanf("%d %d %d", &a, &b, &c);
				add(a, b, -c);//差分边 
				break;
			}
			case 2:
			{
				int a, b, c;
				scanf("%d %d %d", &a, &b, &c);
				add(b, a, c);//权值边 
				break;
			}
			case 3:
			{
				int a, b;
				scanf("%d %d", &a, &b);
				add(a, b, 0);
				add(b, a, 0);//双向边 
				break;
			}
		}
	}
	for (int i = 1; i <= n; i++)
		add(n + 1, i, 0);
	bool flag = spfa(n + 1);//计算最短路径 
	if (!flag) 
	{
		printf("No");
		return 0;
	}
	printf("Yes");
	return 0;
}