方法：后序遍历

    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* removeLeafNodes(TreeNode* root, int target) {
    15.         if(root==NULL){
    16.             return root;
    17.         }
    18.         if(root!=NULL){
    19.             root->left=removeLeafNodes(root->left,target);
    20.             root->right=removeLeafNodes(root->right,target);
    21.             if(root->left==NULL&&root->right==NULL&&root->val==target){
    22.                 root=NULL;
    23.                 }
    24.         }
    25.         return root;
    26.     }
    27. };