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class Solution {
private:
    vector<vector<int>> adj;
    vector<int> g;
    long long ans;
 
    // Returns count of nodes with target group in subtree
    int dfs(int u, int p, int tar, long long tot) {
        // Count current node if it matches
        int cnt = (g[u] == tar) ? 1 : 0;
 
        // Visit all children
        for (int v : adj[u]) {
            if (v == p) continue;
 
            int sub = dfs(v, u, tar, tot);
 
            // Edge cost = one side * other side
            long long rem = tot - sub;
            ans += sub * rem;
 
            cnt += sub;
        }
 
        return cnt;
    }
 
public:
    long long interactionCosts(int n, vector<vector<int>>& edges, vector<int>& group) {
        adj.resize(n);
        g = group;
        ans = 0;
 
        // Build tree
        for (auto &e : edges) {
            adj[e[0]].push_back(e[1]);
            adj[e[1]].push_back(e[0]);
        }
 
        // Count each group size
        vector<int> sz(21, 0);
        for (int x : group) {
            sz[x]++;
        }
 
        // Process each group with 2+ nodes
        for (int i = 1; i <= 20; i++) {
            if (sz[i] >= 2) {
                dfs(0, -1, i, sz[i]);
            }
        }
 
        return ans;
    }
};

prog.cpp:3:5: error: ‘vector’ does not name a type
     vector<vector<int>> adj;
     ^~~~~~
prog.cpp:4:5: error: ‘vector’ does not name a type
     vector<int> g;
     ^~~~~~
prog.cpp:29:39: error: ‘vector’ has not been declared
     long long interactionCosts(int n, vector<vector<int>>& edges, vector<int>& group) {
                                       ^~~~~~
prog.cpp:29:45: error: expected ‘,’ or ‘...’ before ‘<’ token
     long long interactionCosts(int n, vector<vector<int>>& edges, vector<int>& group) {
                                             ^
prog.cpp: In member function ‘int Solution::dfs(int, int, int, long long int)’:
prog.cpp:10:20: error: ‘g’ was not declared in this scope
         int cnt = (g[u] == tar) ? 1 : 0;
                    ^
prog.cpp:13:22: error: ‘adj’ was not declared in this scope
         for (int v : adj[u]) {
                      ^~~
prog.cpp: In member function ‘long long int Solution::interactionCosts(int, int)’:
prog.cpp:30:9: error: ‘adj’ was not declared in this scope
         adj.resize(n);
         ^~~
prog.cpp:31:9: error: ‘g’ was not declared in this scope
         g = group;
         ^
prog.cpp:31:13: error: ‘group’ was not declared in this scope
         g = group;
             ^~~~~
prog.cpp:35:24: error: ‘edges’ was not declared in this scope
         for (auto &e : edges) {
                        ^~~~~
prog.cpp:41:9: error: ‘vector’ was not declared in this scope
         vector<int> sz(21, 0);
         ^~~~~~
prog.cpp:41:16: error: expected primary-expression before ‘int’
         vector<int> sz(21, 0);
                ^~~
prog.cpp:42:22: error: unable to deduce ‘auto&&’ from ‘group’
         for (int x : group) {
                      ^~~~~
prog.cpp:43:13: error: ‘sz’ was not declared in this scope
             sz[x]++;
             ^~
prog.cpp:48:17: error: ‘sz’ was not declared in this scope
             if (sz[i] >= 2) {
                 ^~

Standard output is empty