poj3259

用bellman搜索图的负权边。很巧妙的把物理学的负空间的概念迁移了一下，不过这过程也很自然，不怎么难想，写得很快。

#include "iostream"

using namespace std;

#define inf 1e10

int dis[1005];

struct edge
{
    int s, e, t;
}e[5200];

int n, m, w_m;//number of nodes, number of edges_positive, number of edges_negative

bool bellman(int countt)
{
    bool flag;
    
    for(int i = 0; i < n-1;i++)
    {
        flag = false;
        for(int j = 0; j < countt;j++)
        {
            if(dis[e[j].e] > dis[e[j].s] + e[j].t)
            {
                dis[e[j].e] = dis[e[j].s] + e[j].t;
                flag = true;
            }
        }
        if(!flag) break;
    }
    for(int i = 0; i < countt; i++)
    {
        if (dis[e[i].e] > dis[e[i].s] + e[i].t) {
            return true;
        }
    }
    return false;
}


int main()
{
    int N;
    cin>>N;
    while (N--) {
        memset(dis, inf, sizeof(dis));
        cin>>n>>m>>w_m;
        int countt = 0;
        for(int i = 1; i <= m; i++)
        {
            int u, v, w;
            cin>>u>>v>>w;
            e[countt].s = e[countt+1].e = u;
            e[countt].e = e[countt+1].s = v;//bi-directional
            e[countt++].t = w;
            e[countt++].t = w;
        }
        for(int j = 1; j <= w_m; j++)
        {
            int u, v, w;
            cin>>u>>v>>w;
            e[countt].s = u;
            e[countt].e = v;
            e[countt++].t = -w;//pay attention
        }
        
        if (bellman(countt)) {
            cout << "YES" << endl;
        }
        else cout << "NO" << endl;
    }
}

poj3259

原文：http://blog.csdn.net/zhengnanlee/article/details/18348625