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.

/**
 * 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) return;
        stack<TreeNode*> stk;
        stk.push(root);
        TreeNode np(0);
        TreeNode* ptr = &np;
        while(!stk.empty())
        {
            TreeNode* p = stk.top();
            stk.pop();
            if(p != NULL)
            {
                stk.push(p->right);
                stk.push(p->left);
                
                ptr->right = p;
                ptr = p;
                ptr->left = NULL;
            }
        }
    }
};