<p>Given the <code>root</code>&nbsp;of a binary tree, return all <strong>duplicate subtrees</strong>.</p>

<p>For each kind of duplicate subtrees, you only need to return the root node of any <b>one</b> of them.</p>

<p>Two trees are <strong>duplicate</strong> if they have the <strong>same structure</strong> with the <strong>same node values</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/08/16/e1.jpg" style="width: 450px; height: 354px;" />
<pre>
<strong>Input:</strong> root = [1,2,3,4,null,2,4,null,null,4]
<strong>Output:</strong> [[2,4],[4]]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/08/16/e2.jpg" style="width: 321px; height: 201px;" />
<pre>
<strong>Input:</strong> root = [2,1,1]
<strong>Output:</strong> [[1]]
</pre>

<p><strong class="example">Example 3:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/08/16/e33.jpg" style="width: 450px; height: 303px;" />
<pre>
<strong>Input:</strong> root = [2,2,2,3,null,3,null]
<strong>Output:</strong> [[2,3],[3]]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of the nodes in the tree will be in the range <code>[1, 5000]</code></li>
	<li><code>-200 &lt;= Node.val &lt;= 200</code></li>
</ul>

/**
 * 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:
    vector<TreeNode*> findDuplicateSubtrees(TreeNode* root) {
        vector<TreeNode*>ans;
        unordered_map<string,pair<int,int>>ids;
        // vector<TreeNode*>nodes;
        // dfs(root,nodes);
        // int n=nodes.size();
        // for(int i=0;i<n;i++){
        //     for(int j=i+1;j<n;j++){
        //         if(compare(nodes[i],nodes[j]))
        //             ans.push_back(nodes[i]);
        //     }
        // }
        id(ans,root,ids);
        return ans;
    }
    string id(vector<TreeNode*>&ans,TreeNode* root,unordered_map<string,pair<int,int>>&ids){
        if(!root)return "0";
        string keys=to_string(root->val)+","+id(ans,root->left,ids)+id(ans,root->right,ids);
        auto& p=ids[keys];
        if(p.second++==0){
            p.first=ids.size();
        }else if(p.second==2){
            ans.push_back(root);
        }
        return to_string(p.first);
    }
    void dfs(TreeNode* root,vector<TreeNode*>&nodes){
        if(!root)return;
        nodes.push_back(root);
        dfs(root->left,nodes);
        dfs(root->right,nodes);
    }
    bool compare(TreeNode* r1,TreeNode* r2){
        if(!r1&&!r2)return true;
        if(!r1||!r2)return false;
        if(r1->val!=r2->val)return false;
        return compare(r1->left,r2->left)&&compare(r1->right,r2->right);
    }
};
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