Flatten Binary Tree to Linked List

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

class Solution {public:    TreeNode *node = NULL;    void flatten(TreeNode *root) {        if(root == NULL) return;        if(node != NULL) {            node->left = NULL;            node->right = root;        }        node = root;        TreeNode *left = root->left;        TreeNode *right = root->right;        flatten(left);        flatten(right);    }};