leetcode1110删点成林

    注意特殊情况根节点被删除，就不放入res

     没有解引用&，这个只是复制而已

      删除结点用nullptr而不是NULL

这里用到的是后序遍历

左右子树分别讨论是否为空

写遍历别忘了回溯条件

    /**
    * 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) {}
    * };
    */
    void bitree(vector<TreeNode*>& res,TreeNode*& root,vector<int>& to_delete)
    {
        if(root==NULL)
        return;

    bitree(res,root->left,to_delete);
    bitree(res,root->right,to_delete);
    auto it=find(to_delete.begin(),to_delete.end(),root->val);
    if(it!=to_delete.end())
    {
        if(root->right!=NULL)
        {
            res.push_back(root->right);
        
        }
        if(root->left!=NULL)
        {
            res.push_back(root->left);
        }
  root = nullptr;

    }


    }
    class Solution {
    public:
        vector<TreeNode*> delNodes(TreeNode* root, vector<int>& to_delete) {
        vector<TreeNode*>res;
        if(root)
        {
            auto it=find(to_delete.begin(),to_delete.end(),root->val);
            if(it==to_delete.end())
            {
                res.push_back(root);
            }
        }
            bitree(res,root,to_delete);
            return res;
        }
    };