poj2240（SPFA判环）

最新推荐文章于 2021-09-30 21:00:47 发布

原创最新推荐文章于 2021-09-30 21:00:47 发布 · 422 阅读

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博客探讨了POJ2240题目，重点在于解决SPFA算法在判断环中的应用，提醒注意输出格式应为'Yes'和'No'，避免因大小写错误导致WA。

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反思：题目本身很简单，不过一定要注意输出，这个题目的输出是Yes和No，不是YES和NO，不要WA在这个地方

#include <iostream>
#include <queue>
#include <cstdio>
#include <map>
#include <cstring>
#include <stack>
#include <string>
#include <vector>
#include <cmath>

using namespace std;

const int INF = 1 << 30;
const int maxn = 40;

struct Node
{
    int v;
    double w;
    Node(){}
    Node(int vv, double ww) : v(vv), w(ww) {}
};

int n, m;
vector<vector<Node> > G;
map<string, int> ind;
double dis[maxn];
int update[maxn];

void Init()
{
    memset(update, 0, sizeof(update));
    for (int i = 1; i <= n; i++) {
        dis[i] = 0;
    }
    G.resize(n + 1);
    for (int i = 1; i <= n; i++) {
        G[i].clear();
    }
}

int SPFA(int v)
{
    queue<int> q;
    q.push(v);
    dis[v] = 1;
    while (!q.empty()) {
        int t = q.front();
        q.pop();
        for (int i = 0, j = G[t].size(); i < j; i++) {
            Node tmp = G[t][i];
            //cout << tmp.v << endl;
            double md = tmp.w * dis[t];
            if (md > dis[tmp.v]) {
                dis[tmp.v] = md;
                update[tmp.v]++;
                if (update[tmp.v] >= n) {
                    return 1;
                }
                q.push(tmp.v);
            }
        }
    }
    return 0;
}

int main()
{
    #ifndef ONLINE_JUDGE
    freopen("in.txt", "r", stdin);
    //freopen("out.txt", "w", stdout);
    #endif // ONLINE_JUDGE
    int kase = 1;
    while (cin >> n) {
        if (n == 0) break;
        Init();
        string s;
        for (int i = 1; i <= n; i++) {
            cin >> s;
            ind[s] = i;
        }
        cin >> m;
        string s1, s2;
        double r;
        for (int i = 0; i < m; i++) {
            cin >> s1 >> r >> s2;
            int a1 = ind[s1];
            int a2 = ind[s2];
            G[a1].push_back(Node(a2, r));
        }
        int flag = SPFA(1);
        cout << "Case " << kase++ << ": ";
        if (flag) {
            cout << "Yes" << endl;
        } else {
            cout << "No" << endl;
        }
    }
    return 0;
}