最短路 || POJ 1847 Tram

POJ 1847 最短路

每个点都有初始指向，问从起点到终点最少要改变多少次点的指向

*初始指向的那条边长度为0，其他的长度为1，表示要改变一次指向，然后最短路

=========高亮！！！==========数组要开n^2的QAQ

#include <iostream>
#include <cstdio>
#include <queue>
using namespace std;
#define SZ 10005//开n^2的数组！！
#define INF 1e9+10
int head[SZ], nxt[SZ], tot = ;
struct edge
{
    int t, d;
}l[SZ];
void build(int f, int t, int d)
{
    l[++tot] = (edge){t, d};
    nxt[tot] = head[f];
    head[f] = tot;
}
int dist[SZ];
bool use[SZ];
queue<int> q;
int spfa(int s, int e)
{
    for(int i = ; i < SZ; i++) dist[i] = INF, use[i] = ;
    while(!q.empty()) q.pop();
    use[s] = ;
    dist[s] = ;
    q.push(s);
    while(!q.empty())
    {
        int u = q.front(); q.pop();
        use[u] = ;
        for(int i = head[u];i;i = nxt[i])
        {
            int v = l[i].t;
            if(dist[v] > dist[u] + l[i].d)
            {
                dist[v] = dist[u] + l[i].d;
                if(!use[v])
                    use[v] = , q.push(v);
            }
        }
    }
    return dist[e];
}
int main()
{
    int n, a, b;
    scanf("%d %d %d", &n, &a, &b);
    for(int i = ; i <= n; i++)
    {
        int k, x;
        scanf("%d", &k);
        if(k == ) continue;//考虑k==0的情况
        scanf("%d", &x);
        build(i, x, );
        for(int j = ; j < k; j++)
        {
            scanf("%d", &x);
            build(i, x, );
        }
    }
    int dis = spfa(a, b);
    if(dis == INF) printf("-1\n");
    else printf("%d\n", dis);
    return ;
}