[Leetcode 114, Medium] 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.

Analysis:

Solutions:

C++:

    void flatten(TreeNode* root) 
    {
        if(root == NULL)
            return;

        if(root->left)
            flatten(root->left);
            
        if(root->right)
            flatten(root->right);
        
        if(root->left) {
            TreeNode *temp_left = root->left;
            root->left = NULL;
            TreeNode *temp_right = root->right;
            root->right = temp_left;
            TreeNode *p_cur = root->right;
            for(; p_cur->right != NULL; p_cur = p_cur->right) {}
            p_cur->right = temp_right;
        }
    }

Java:

Python: