Flatten Binary Tree to Linked List

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void flatten(TreeNode* root) {
        if(!root)
            return;
        
        flatten(root->left);
        flatten(root->right);
        
        if(root->left == nullptr)
            return;
        
        TreeNode *cur = root->left;
        while(cur->right)
            cur = cur->right;
        cur->right = root->right;
        root->right = root->left;
        root->left = nullptr;
    }
};

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void flatten(TreeNode* root) {
        if(!root)
            return;
            
        stack<TreeNode *> s;
        s.push(root);
        
        while(!s.empty())
        {
            TreeNode *tmp = s.top();
            s.pop();
            
            if(tmp->right)
                s.push(tmp->right);
            if(tmp->left)
                s.push(tmp->left);
            
            if(!s.empty())
                tmp->right = s.top();
            tmp->left = nullptr;
        }
    }
};