POJ2983 Is the Information Reliable?

经典的差分约束问题，对于每一条语句：

P A B X  ==>  dis[B]-dis[A]==X  ==>  dis[B]-dis[A]>=X && dis[A]-dis[B]>=X

V A B  ==>  dis[B]-dis[A]>=1

然后相应的连边即可，最后用spfa判断负环，若无负环，则输出Reliable，反之Unreliable

注意：spfa中的松弛操作应作相应修改，还有spfa无法处理不连通图，可以建立超级源点，或者事先将所有点入队

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <climits>
#include <cmath>
#include <iostream>
#include <string>
#include <vector>
#include <set>
#include <map>
#include <list>
#include <queue>
#include <stack>
#include <deque>
#include <algorithm>
using namespace std;

struct Edge
{
    int from,to;
    int d;
    Edge(int from, int to, int d) {this->from=from; this->to=to; this->d=d;}
};

const int INF = 0x3f3f3f3f;
const int maxn = 1010;
int n,m;
int dis[maxn];
int cnt[maxn];
bool vis[maxn];
vector<Edge> edges;
vector<int> g[maxn];
bool ok;

void init()
{
    for (int i=0;i<n;i++)
        g[i].clear();
    edges.clear();
}

void addEdge(int from,int to,int d)
{
    edges.push_back(Edge(from,to,d));
    int t=edges.size();
    g[from].push_back(t-1);
}

bool spfa(int st)
{
    for (int i=0;i<n;i++)
        dis[i]=INF;
    dis[st]=0;
    for (int i=0;i<n;i++)
        vis[i]=0;
    for (int i=0;i<n;i++)
        cnt[i]=0;
    queue<int> Q;
    while (!Q.empty()) Q.pop();
    Q.push(st);
    vis[st]=1;
    cnt[st]=1;
    while (!Q.empty())
    {
        int u = Q.front();
        for(unsigned int i=0;i<g[u].size();i++)
        {
			Edge &e=edges[g[u][i]];
			if (dis[e.to]==INF || dis[u]+e.d>dis[e.to])
			{
			    dis[e.to]=dis[u]+e.d;
				if(!vis[e.to])
				{
					Q.push(e.to);
                    vis[e.to]=1;
                    cnt[e.to]++;
					if(cnt[e.to]>=n) //已经超过n次,出现负环
                        return 0;
				}
			}
		}
		Q.pop();
		vis[u]=0;
    }
    return 1;
}

int main()
{
    int a,b,c;
    char ch[3];
    while (scanf("%d%d",&n,&m)==2)
    {
        n++;
        ok=1;
        init();
        for (int i=1;i<n;i++) addEdge(0,i,0);
        while (m--)
        {
            scanf("%s",ch);
            if (ch[0]=='P')
            {
                scanf("%d%d%d",&a,&b,&c);
                addEdge(a,b,c);
                addEdge(b,a,-c);
                if (a==b && c!=0) ok=0;
            }
            else
            {
                scanf("%d%d",&a,&b);
                addEdge(a,b,1);
                if (a==b) ok=0;
            }
        }
        if (spfa(0) && ok) puts("Reliable");
        else puts("Unreliable");
    }
	return 0;
}

第一次用codeblocks的default code功能，自动加了一堆头文件，写在blog里还是有点影响观赏的= =