1function TreeNode(val) {
2    this.val = val;
3    this.left = this.right = null;
4}
5
6var findDuplicateSubtrees = function(root) {
7    const map = new Map();
8    function serialize(node) {
9        if (!node) return '#';
10        const serial = `${node.val},${serialize(node.left)},${serialize(node.right)}`;
11        if (!map.has(serial)) {
12            map.set(serial, []);
13        }
14        map.get(serial).push(node);
15        return serial;
16    }
17    serialize(root);
18    const result = [];
19    for (const nodes of map.values()) {
20        if (nodes.length > 1) {
21            result.push(nodes[0]);
22        }
23    }
24    return result;
25};

1#include <unordered_map>
2#include <vector>
3
struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution {
    unordered_map<string, int> ids;
    unordered_map<string, int> count;
    vector<TreeNode*> result;
    int idCounter = 1;
public:
    vector<TreeNode*> findDuplicateSubtrees(TreeNode* root) {
        serialize(root);
        return result;
    }
    
private:
    int serialize(TreeNode* node) {
        if (!node) return 0;
        string serial = to_string(node->val) + ',' + to_string(serialize(node->left)) + ',' + to_string(serialize(node->right));
        if (!ids.count(serial)) {
            ids[serial] = idCounter++;
        }
        int id = ids[serial];
        if (++count[serial] == 2) {
            result.push_back(node);
        }
        return id;
    }
};