652. 寻找重复的子树

题目：652. 寻找重复的子树
思路：二叉树序列化

/**
 * 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) {
        dfs(root);
        return res;
    }
private:
    int idx = 0;
    vector<TreeNode*> res;
    unordered_map<string,pair<int,bool>> visited;
    int dfs(TreeNode* root){
        if(root == nullptr) return 0;
        string key = to_string(root->val) + "," + to_string(dfs(root->left)) + "," + to_string(dfs(root->right));
        if( visited.find(key)!= visited.end()){   //find
            if(visited[key].second == false){
                visited[key].second = true;
                res.emplace_back(root);
            }
        }else{
            visited[key] = {++idx, false};
        }
        return visited[key].first;
    }
};