POJ1860 SPFA

2017年3月25日 | ljfcnyali
题目大意
给定N种货币，某些货币之间可以相互兑换，现在给定一些兑换规则，问能否从某一种货币开始兑换，经过一些中间货币之后，最后兑换回这种货币，并且得到的钱比之前的多。

Sample Input

3 2 1 20.01 2 1.00 1.00 1.00 1.002 3 1.10 1.00 1.10 1.00

Sample Output

YES

1
YES
题目分析
使用SPFA判断负权环，一个一个乱搞就可以了。
AC代码

/*************************************************************************    > File Name: POJ1860.cpp    > Author: ljf-cnyali    > Mail: [e-mail]     > Created Time: 2017/3/25 15:00:46 ************************************************************************/#include<iostream>#include<cstdio>#include<cstdlib>#include<cmath>#include<cstring>#include<algorithm>#include<map>#include<set>#include<vector>#include<queue>using namespace std;#define REP(i, a, b) for(int i = (a), _end_ = (b);i <= _end_; ++ i)#define mem(a) memset((a), 0, sizeof(a))#define str(a) strlen(a)const int maxn = 210;int n, m, S;int begin[maxn], next[maxn], to[maxn], e, vis[maxn], Num[maxn];double V, r[maxn], c[maxn], dist[maxn];void add(int x, int y, double a, double b) {    to[++ e] = y;    next[e] = begin[x];    begin[x] = e;    r[e] = a;    c[e] = b;}bool SPFA() {    queue<int> Q;    vis[S] = 1;    dist[S] = V;    Q.push(S);    Num[S] ++;    while(!Q.empty()) {        int p = Q.front();        Q.pop();        vis[p] = 0;        for(int i = begin[p]; i; i = next[i]) {            int q = to[i];            if(dist[q] < (dist[p] - c[i]) * r[i]) {                dist[q] = (dist[p] - c[i]) * r[i];                if(!vis[q]) {                    vis[q] = 1;                    Q.push(q);                    Num[q] ++;                    if(Num[q] > n)                        return true;                }            }        }    }    if(dist[S] > V)        return true;    return false;}int main() {#ifndef ONLINE_JUDGE    freopen("input.txt", "r", stdin);    freopen("output.txt", "w", stdout);#endif    int x, y;    double r1, c1, r2, c2;    while(scanf("%d%d%d%lf", &n, &m, &S, &V) != EOF) {        e = 0;        mem(begin);mem(next);mem(to);mem(vis);mem(r);mem(c);        REP(i, 1, m) {            scanf("%d%d%lf%lf%lf%lf", &x, &y, &r1, &c1, &r2, &c2);            add(x, y, r1, c1);            add(y, x, r2, c2);        }        if(SPFA())            printf("YES\n");        else            printf("NO\n");    }    return 0;}

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