HDU4460(最短路)

#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <map>
#include <vector>
#include <queue>
using namespace std;
#define maxn 1111
#define maxm 21111
#define INF 111111111

struct node {
    int v, w;
    node (int v = 0, int w = 0) : v(v), w(w) {}
};
vector <node> edge[maxn];
int n, cnt[maxn];
bool vis[maxn];
string name, name2;
map <string, int> gg;
int ans, cur;
int d[maxn];
int fa[maxn];

int find (int x) {
    return fa[x] == x ? fa[x] : fa[x] = find (fa[x]);
}

void add_edge (int from, int to, int w) {
    edge[from].push_back (node (to, w));
}

bool spfa (int start) {
    memset (vis, 0, sizeof vis);
    for (int i = 1; i <= n; i++)
        d[i] = INF;
    vis[start] = 1; d[start] = 0;
    queue <int> que;
    while (!que.empty ())
        que.pop ();
    memset (cnt, 0, sizeof cnt);
    cnt[start] = 1;
    que.push (start);
    while (!que.empty ()) {
        int u = que.front (); que.pop ();
        vis[u] = 0;
        for (int i = 0; i < edge[u].size (); i++) {
            int v = edge[u][i].v; 
            if (d[v] > d[u]+edge[u][i].w) {
                d[v] = d[u]+edge[u][i].w;
                if (!vis[v]) {
                    vis[v] = 1;
                    que.push (v);
                    if (++cnt[v] > n)
                        return 0;
                }
            }
        }
    }
    cur = 0;
    for (int i = 1; i <= n; i++) {
        cur = max (cur, d[i]);
    }
    ans = max (cur, ans);
    return 1;
}

int main () {
    //freopen ("in", "r", stdin);
    while (scanf ("%d", &n) == 1 && n) {
        gg.clear ();
        for (int i = 1; i <= n; i++)
            fa[i] = i, edge[i].clear ();
        for (int i = 1; i <= n; i++) {
            cin >> name;
            gg[name] = i;
        }
        int q;
        scanf ("%d", &q);
        while (q--) {
            cin >> name >> name2;
            int u = gg[name], v = gg[name2];
            add_edge (u, v, 1);
            add_edge (v, u, 1);
            int p1 = find (u), p2 = find (v);
            if (p1 != p2) {
                fa[p1] = p2;
            }
        }
        int kk = find (1);
        for (int i = 2; i <= n; i++) {
            if (find (i) != kk) {
                printf ("-1\n");
                goto out;
            }
        }
        ans = 0;
        for (int i = 1; i <= n; i++)
            spfa (i);
        printf ("%d\n", ans);
        out: ;
    }
    return 0;
}