nyoj Arbitrage (Bellman-Ford)

跟天下第一挺像的

void Bellman(int v0)
{
    int i, k;
    for (int i = 0; i < n; ++i) {
        dist[i] = INF, path[i] = -1;
    }
    dist[v0] = 0;
    for (k = 1; k < n; ++k) {
        for (i = 0; i < m; ++i) {
            if (dist[edges[i].u] != INF && edges[i].w + dist[edges[i].u] < dist[edges[i].v]) {
                dist[edges[i].v] = edges[i].w + dist[edges[i].u];
                path[edges[i].v] = edges[i].u;
            }
        }
    }
}

//判断是否存在从原点可达的负权值回路
for (i = 0; i < m; ++i) {
    if (dist[edges[i].u] != INF && edges[i].w + dist[edges[i].u] < dist[edges[i].v])
        return 0;
}
return 1;

 该算法还可以求最长路径

Bellman-Ford 判断环 + 最大路径值是否大于1

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<queue>
#include<algorithm>
#include<map>
#include<iomanip>
#include<climits>
#include<string.h>
#include<cmath>
#include<stdlib.h>
#include<vector>
#include<set>
#define INF 1e7
#define MAXN 100010
#define maxn 50
#define maxm 1000
#define Mod 1000007
using namespace std;
typedef long long LL;

struct exchange{
    int ci, cj;
    double cij;
}ex[maxm];
int i, j, k;
int n, m;
string name[maxn], a, b;
double x, maxdist[maxn];
bool flag;
int kase = 0;

int readkase()
{
    cin >> n;
    if (n == 0) return 0;
    for (i = 0; i < n; ++i)
        cin >> name[i];
    cin >> m;
    for (i = 0; i < m; ++i) {
        cin >> a >> x >> b;
        for (j = 0; a != name[j]; ++j);
        for (k = 0; b != name[k]; ++k);
        ex[i].ci = j, ex[i].cj = k, ex[i].cij = x;
    }
    return 1;
}

void Bellman(int v0)
{
    flag = false;
    memset(maxdist,0,sizeof(maxdist));
    maxdist[v0] = 1;
    for (k = 1; k <= n; ++k) {   //从maxdist(0)递推到maxdist(n)
        for (i = 0; i < m; ++i) {//判断加入每条边是否能是最大距离增大
            if (maxdist[ex[i].ci] * ex[i].cij > maxdist[ex[i].cj])
                maxdist[ex[i].cj] = maxdist[ex[i].ci] * ex[i].cij;
        }
    }
    if (maxdist[v0] > 1.0) flag = true;
}

int main()
{
    while (readkase()) {
        for (int i = 0; i < n; ++i) {
            Bellman(i);
            if (flag) break;
        }
        if (flag) printf("Case %d: Yes
",++kase);
        else printf("Case %d: No
", ++kase);
    }
    return 0;
}