遇到问题：delete使用，需要查资料

    1. /**
    2.  * Definition for a binary tree node.
    3.  * struct TreeNode {
    4.  *     int val;
    5.  *     TreeNode *left;
    6.  *     TreeNode *right;
    7.  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
    8.  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    9.  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
    10.  * };
    11.  */
    12. class Solution {
    13. public:
    14.     TreeNode* pruneTree(TreeNode* root) {
    15.         if(root==NULL){
    16.             return root;
    17.         }
    18.         if(!check(root)){
    19.             root=NULL;
    20.         }
    21.         if(root!=NULL){
    22.             TreeNode* left1=pruneTree(root->left);
    23.             TreeNode* right1=pruneTree(root->right);
    24.             root->left=left1;
    25.             root->right=right1;
    26.         }
    28.         return root;
    29.     }
    30.     bool check(TreeNode* root){
    31.         if(root==NULL){
    32.             return false;
    33.         }
    34.         if(root->val==1){
    35.             return true;
    36.         }
    37.         bool a1=check(root->left);
    38.         bool a2=check(root->right);
    39.         return a1||a2;
    40.     }
    41. };