A.基站

建出MST即可，但先要用dijkstra求出每个点到达的最近基站。
可预先将所有基站放入堆中迭代。

// Program written by Liu Zhaozhou ~~~
#include <bits/stdc++.h>
#include <tr1/unordered_map>
#define lowbit(x) (x & -x)

using namespace std;

inline char gc(void) {
    static char buf[100000], *p1 = buf, *p2 = buf;
    return p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, 100000, stdin), p1 == p2) ? EOF : *p1++;
}

#define gc() getchar()

template <class T> inline void read(T &x) {
    T f = 1; x = 0; static char c = gc();
    for (; !isdigit(c); c = gc()) if (c == '-') f = -f;
    for (; isdigit(c); c = gc()) x = (x << 1) + (x << 3) + (c & 15);
    x *= f;
}

inline void readstr(char *s) {
    do *s = gc(); while ((*s == ' ') || (*s == '\n') || (*s == '\r'));
    do *(++s) = gc(); while ((~*s) && (*s != ' ') && (*s != '\n') && (*s != '\r'));
    *s = 0; return;
}

inline void readch(char&x) { while (isspace(x = gc())); }

template <class T> inline void write(T x) {
    if (x < 0) x = -x, putchar('-');
    if (x > 9) write(x / 10);
    putchar(x % 10 + 48);
}

template <class T> inline void writeln(T x, char c = '\n') { write(x); putchar(c); }
template <class T> inline void chkmax(T &x, const T y) { x > y ? x = x : x = y; }
template <class T> inline void chkmin(T &x, const T y) { x < y ? x = x : x = y; }

#define Ms(arr, opt) memset(arr, opt, sizeof(arr))
#define Mp(x, y) make_pair(x, y)

typedef long long ll;

const int Maxn = 1e5 + 5, Maxm = 1e6 + 5;
int n, m, k, cnt = 0, head[Maxn], ver[Maxm], nxt[Maxm], edge[Maxm];
inline void AddEdge(int u, int v, int w) {
    ver[++cnt] = v, edge[cnt] = w, nxt[cnt] = head[u], head[u] = cnt;
    ver[++cnt] = u, edge[cnt] = w, nxt[cnt] = head[v], head[v] = cnt;
}

struct state {
    int u, d;
    state(void) { u = d = 0; }
    state(int _u, int _d) : u(_u), d(_d) {}
    inline bool operator < (const state&rhs) const { return d > rhs.d; }
};

int c[Maxn], nrc[Maxn], dis[Maxn]; bool vis[Maxn];
inline void dijkstra(void) {
    priority_queue <state> q; Ms(dis, 0x3f);
    for (int i = 1; i <= k; i++)
        dis[c[i]] = 0, nrc[c[i]] = c[i], q.push(state(c[i], 0));
    while (!q.empty()) {
        int u = q.top().u; q.pop();
        if (vis[u]) continue; vis[u] = true;
        for (int i = head[u]; i; i = nxt[i]) {
            if (dis[ver[i]] > dis[u] + edge[i]) {
                dis[ver[i]] = dis[u] + edge[i];
                nrc[ver[i]] = nrc[u]; q.push(state(ver[i], dis[ver[i]]));
            }
        }
    }
}

struct Edge {
    int u, v, w;
    Edge(void) { u = v = w = 0; }
    Edge(int _u, int _v, int _w) : u(_u), v(_v), w(_w) {}
    inline bool operator < (const Edge&rhs) const { return w < rhs.w; }
} e[Maxm];

int f[Maxn];
inline int getf(int x) { return f[x] == x ? x : f[x] = getf(f[x]); }

signed main(void) {
    read(n), read(m);
    for (int i = 1, u, v, w; i <= m; i++)
        read(u), read(v), read(w), AddEdge(u, v, w);
    read(k); for (int i = 1; i <= k; i++) read(c[i]);

    dijkstra(); int tot = 0;
    for (int u = 1; u <= n; u++)
    for (int i = head[u]; i; i = nxt[i])
    if (nrc[u] != nrc[ver[i]])
        e[++tot] = Edge(nrc[u], nrc[ver[i]], dis[u] + dis[ver[i]] + edge[i]);

    sort(e + 1, e + tot + 1); int ans = 0;
    for (int i = 1; i <= n; i++) f[i] = i;
    for (int i = 1; i <= tot; i++) {
        int u = getf(e[i].u), v = getf(e[i].v), w = e[i].w;
        if (u != v) f[u] = v, ans = w;
    } writeln(ans);
    return 0;
}

/**/