LeetCode 114. 二叉树展开为链表 C++

/**
 * 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:
    void flatten(TreeNode* root) {
        TreeNode*last = NULL;
        preorder(root,last);

    }
    void preorder(TreeNode* node,TreeNode* &last){
        if(!node) return;
                if( !node->right &&!node->left){
            last = node;
            return;
        }TreeNode* left = node->left;
        TreeNode* right = node->right;
        TreeNode* left_last = NULL;
        TreeNode* right_last = NULL;

        if(left){
            preorder(left,left_last);
            node->left = NULL;
            node->right = left;
            last = left_last;
        }
        if(right){
            preorder(right,right_last);
            if(left_last){
                left_last->right = right;
            }
            last = right_last;
        }

    }
};