# Description

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
``````

The original problem is here.

The original code is here.

# My Solution

I solve this problem in C++, as below:

``````/*
*Flatten Binary Tree to Linked List
*Author: shuaijiang
*Email: zhaoshuaijiang8@gmail.com
*/
#include<iostream>
#include<stack>
#include<stdlib.h>
using namespace std;

/**
* 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) {
stack<TreeNode*> myStack;
if(root==NULL)
return;

if(root->right!=NULL)
myStack.push(root->right);
if(root->left!=NULL)
myStack.push(root->left);
root->left = NULL;
root->right= NULL;
TreeNode * lastNode = root;
while(!myStack.empty()){
TreeNode * node = myStack.top();
myStack.pop();
if(node->right!=NULL)
myStack.push(node->right);
if(node->left!=NULL)
myStack.push(node->left);
node->left=NULL;
node->right=NULL;
lastNode->right=node;
lastNode->left =NULL;

lastNode = lastNode->right;
}
}
};
``````

# Note

In order to flatten the binary tree, we need to traversal the tree by pre-order and put the all child nodes to right of the father node.