POJ 3169 Layout

裸差分约束。有两种写法。spfa 和 dfs，dfs 优点在判负环，但递归常数巨大，spfa 优点在快。
差分约束就是用最短路的思想来接不等式组，具体做法见这篇博客。

#include <bits/stdc++.h>
using namespace std;

#define db double
#define ll long long
#define RG register

inline int gi()
{
	RG int ret; RG bool flag; RG char ch;
	ret=0, flag=true, ch=getchar();
	while (ch < '0' || ch > '9')
		ch == '-' ? flag=false : 0, ch=getchar();
	while (ch >= '0' && ch <= '9')
		ret=(ret<<3)+(ret<<1)+ch-'0', ch=getchar();
	return flag ? ret : -ret;
}

const db pi = acos(-1.0);
const int N = 1005, M = 2e4+5, inf = 1<<30;

bool vis[N];
int f[N],dis[N],et[M],nx[M],ed[M];
int num;

inline void add(int x,int y,int z)
{
	nx[++num]=f[x], et[num]=y, ed[num]=z, f[x]=num;
}

inline void spfa(int o)
{
	int i,to,len;
	vis[o]=true;
	for (i=f[o]; i; i=nx[i])
		{
			to=et[i], len=ed[i];
			if (dis[to] > dis[o]+len)
				{
					if (vis[to])
						puts("-1"), exit(0);
					dis[to]=dis[o]+len;
					spfa(to);
				}
		}
	vis[o]=false;
}

int main()
{
	freopen("layout.in","r",stdin);
	freopen("layout.out","w",stdout);
	int n,m,k,i,x,y,z;
	n=gi(), m=gi(), k=gi();
	for (i=1; i<=m; ++i)
		{
			x=gi(), y=gi(), z=gi();
			add(x,y,z);
		}
	for (i=1; i<=k; ++i)
		{
			x=gi(), y=gi(), z=gi();
			add(y,x,-z);
		}
	for (i=2; i<=n; ++i)
		dis[i]=inf;
	spfa(1);
	dis[n] < inf ? printf("%d\n",dis[n]) : puts("-2");
	return 0;
}