> For the complete documentation index, see [llms.txt](https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jimmylin1991.gitbook.io/practice-of-algorithm-problems/tree/156.-binary-tree-upside-down.md).

# 156. Binary Tree Upside Down

Given a binary tree where all the right nodes are either leaf nodes with a sibling (a left node that shares the same parent node) or empty, flip it upside down and turn it into a tree where the original right nodes turned into left leaf nodes. Return the new root.

**Example:**

```
Input: [1,2,3,4,5]

    1
   / \
  2   3
 / \
4   5

Output: return the root of the binary tree [4,5,2,#,#,3,1]

   4
  / \
 5   2
    / \
   3   1  
```

**Clarification:**

Confused what `[4,5,2,#,#,3,1]` means? Read more below on how binary tree is serialized on OJ.

The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.

Here's an example:

```
   1
  / \
 2   3
    /
   4
    \
     5
```

The above binary tree is serialized as `[1,2,3,#,#,4,#,#,5]`.

{% hint style="info" %}
這題有點像是每個root, left child, right child順時針旋轉。
{% endhint %}

```cpp
// Recursion Method
TreeNode* upsideDownBinaryTree(TreeNode* root) { // time: O(n); space: O(n)
    if (!root || !root->left) return root;
    TreeNode *l = root->left, *r = root->right;
    TreeNode *newRoot = upsideDownBinaryTree(l);
    l->left = r; // original right node becomes left node
    l->right = root; // original root becomes right node
    root->left = nullptr;
    root->right = nullptr;
    return newRoot;
}
```

{% hint style="info" %}
next: 當前這輪的left node是下一輪的current node\
pre: 當前這輪的current node是下一輪的right node\
tmp: 當前這輪的right node是下一輪的left node
{% endhint %}

```cpp
// Iteration Method
TreeNode* upsideDownBinaryTree(TreeNode* root) { // time: O(n); space: O(1)
    TreeNode *cur = root, *next = nullptr, *pre = nullptr, *tmp = nullptr;
    while (cur) {
        next = cur->left; // the next iteration cur node
        cur->left = tmp;
        tmp = cur->right; // current right node is left node in the next iteration
        cur->right = pre;
        pre = cur; // cur root node is the right node in the next iteration
        cur = next;
    }
    return pre;
}
```