Flatten Binary Tree to Linked List [Leetcode]

Given a binary tree, flatten it to a linked list in-place.
For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6

The flattened tree should look like:

   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

Solution: two ways to solve this problem recursively. The first way is: flatten the left and right subtrees first, and then append the flattened right subtree to the right child of the flattened  left subtree; at the very last append the head of the flattened left subtree to the right child of the root and set the left child of the root to null; The second way is, as noticed, each node's right child points to the next node of a pre-order traversal in the flattened tree.

   /*******************************SOLUTION 1***********************************/

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void flatten(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if((root==NULL) || (root->left==NULL&&root->right==NULL))
            return;
        TreeNode* rchild = root->right;
        TreeNode* ltail = NULL;
        if(root->left!=NULL){
            flatten(root->left);
            root->right = root->left;
            ltail = root->left;
            while(ltail->right!=NULL)
                ltail = ltail->right;
        }
        root->left = NULL;
        if(rchild!=NULL){
            flatten(rchild);
            if(ltail!=NULL){
                ltail->right = rchild;
            }
            else
                root->right = rchild;
        }
    }
}; 

   /*******************************SOLUTION 2***********************************/

class Solution {
public:
    void flattenHelper(TreeNode* root, TreeNode* &head){
        if(root!=NULL){
            flattenHelper(root->right, head);
            flattenHelper(root->left, head);
            root->right = head;
            root->left = NULL;
            head = root;
        }
    }
    void flatten(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        TreeNode* head = NULL;
        flattenHelper(root, head);
    }
};