Flatten Binary Tree to Linked List Solutions in C++

// Source : https://oj.leetcode.com/problems/flatten-binary-tree-to-linked-list/
// Author : Hao Chen
// Date   : 2014-07-03



/**
 * 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) {
        
        vector<TreeNode*> v, stack;
        stack.push_back(root);
        while(stack.size()>0){
            TreeNode* node = stack.back();
            stack.pop_back();
            v.push_back(node);
            
            if (node && node->right){
                stack.push_back(node->right);
            }
            if (node && node->left){
                stack.push_back(node->left);
            }
        }
        
        v.push_back(NULL);
        for(int i=0; i<v.size(); i++){
            if (v[i]){
                v[i]->left = NULL;
                v[i]->right = v[i+1];
            }
        }
        
    }
};

#include <cstddef>

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution {
public:
    void flatten(TreeNode *root){
        for ( ; root; root = root->right)
            if (root->left) {
                TreeNode *end = root->left;
                while (end->right) end = end->right;
                end->right = root->right;
                root->right = root->left;
                root->left = NULL;
            }
    }
};