Binary Tree Pruning

给一个二叉树, 求删除值为0并且左右两边都是nullptr的分支.

/**
 * 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* pruneTree(TreeNode* root) {
        if(!root)
            return root;
        root->left = pruneTree(root->left);
        root->right = pruneTree(root->right);
        if(!root->val && !root->left && !root->right)
            return nullptr;
        else
            return root;
    }
};