LeetCode Flatten Binary Tree To Linked List

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.