DeleteNodes&ReturnForest.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:
    TreeNode* postorder(TreeNode* &root,unordered_set<int>& st,vector<TreeNode*>& res)
    {
        if(root==NULL)
            return NULL;
        root->left=postorder(root->left,st,res);
        root->right=postorder(root->right,st,res);
        if(st.count(root->val))
        {
            if(root->left)
                res.push_back(root->left);
            if(root->right)
                res.push_back(root->right);
            root = NULL;
        }
        return root;
     //   postorder(root->left,st,res);
     //   postorder(root->right,st,res);
    }
    vector<TreeNode*> delNodes(TreeNode* root, vector<int>& to_delete) {
     unordered_set<int>st(to_delete.begin(),to_delete.end());
        vector<TreeNode*>res;
        postorder(root,st,res);
        if(root)
            res.push_back(root);
        return res;
    }
};