LeetCode 114. 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

it is actually not to flatten into a linked list but a linked list like tree.

TreeNode* goRightMost(TreeNode* root) {
    if(!root) return NULL;
    TreeNode* tmp = root;
    while(tmp->right) {
        tmp = tmp->right;
    }
    return tmp;
}

void flatten(TreeNode* root) {
    if(!root) return;
    TreeNode* tmp = root;
    while(tmp) {
        if(tmp->left) {
            TreeNode* rightMost = goRightMost(tmp->left);
            rightMost->right = tmp->right;
            tmp->right = tmp->left;
            tmp->left = NULL;
        }
        tmp = tmp->right;
    }
}