LeetCode Problem : Flatten Binary Tree to Linked List

Problem

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
click to show hints.

Hints:
If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

Code

void flatten_r(TreeNode *root,TreeNode **last) {
    if(!root)
        return;
    if(*last){
        (*last)->right = root;
        (*last)->left = 0;
    }
    TreeNode *l = root->left;
    TreeNode *r = root->right;
    *last = root;
    flatten_r(l,last);
    flatten_r(r,last);
}
void flatten(TreeNode *root) {
    // IMPORTANT: Please reset any member data you declared, as
    // the same Solution instance will be reused for each test case.
    TreeNode *last = 0;
    flatten_r(root,&last);
}