Code

#include <bits/stdc++.h>
using namespace std;

const int inf = 1e7;

void solve() {

    int n, m, k;
    cin >> n >> m >> k;

    vector<vector<pair<int, int>>> adj(n);
    for (int i = 0; i < m; i++) {
        int u, v, w;
        cin >> u >> v >> w;
        u--;
        v--;
        adj[u].push_back(make_pair(v, w));
        adj[v].push_back(make_pair(u, w));
    }

    vector<bool> has_switch(n, false);
    for (int i = 0; i < k; i++) {
        int index;
        cin >> index;
        index--;

        has_switch[index] = true;
    }

    // Each vertex in state graph is identified by (vertex, flip_cnt%2)
    vector<vector<int>> level(n, vector<int>(2, inf));

    queue<pair<int, int>> bfs;
    level[0][0] = 0;
    bfs.push(make_pair(0, 0));

    if (has_switch[0]) {
        level[0][1] = 0;
        bfs.push(make_pair(0, 1));
    }

    while (!bfs.empty()) {
        int vertex_now = bfs.front().first;
        int flip_cnt = bfs.front().second;
        int dist_now = level[vertex_now][flip_cnt];
        bfs.pop();

        for (auto child : adj[vertex_now]) {
            // Don't use the switch. In that case, an edge would only be
            // iterable if the the xor of flip_cnt and edge weight is 1.
            if (child.second ^ flip_cnt == 1) {
                int nxt = child.first;

                // Don't use the switch.
                if (level[nxt][flip_cnt] == inf) {
                    level[nxt][flip_cnt] = dist_now + 1;
                    bfs.push(make_pair(nxt, flip_cnt));
                }
            }

            // Now, use the switch at this vertex if possible.
            if (child.second ^ flip_cnt == 0) {
                int nxt = child.first;

                // Use the switch.
                if (has_switch[vertex_now] && level[nxt][flip_cnt ^ 1] == inf) {
                    level[nxt][flip_cnt ^ 1] = dist_now + 1;
                    bfs.push(make_pair(nxt, flip_cnt ^ 1));
                }
            }
        }
    }

    int res = min(level[n - 1][0], level[n - 1][1]);
    if (res >= inf) {
        cout << -1 << endl;
    } else {
        cout << res << endl;
    }
}

int main() {
    solve();
    return 0;
}