Solution.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    unordered_map<string, int> counter;
    vector<TreeNode*> ans;

    vector<TreeNode*> findDuplicateSubtrees(TreeNode* root) {
        dfs(root);
        return ans;
    }

    string dfs(TreeNode* root) {
        if (!root) return "#";
        string v = to_string(root->val) + "," + dfs(root->left) + "," + dfs(root->right);
        ++counter[v];
        if (counter[v] == 2) ans.push_back(root);
        return v;
    }
};