# Leetcode Invert-Binary-Tree

#### 2022-07-06 13:33

> ##### Algorithms:
>
> #algorithm #DFS #recursion
>
> ##### Data structures:
>
> #DS #binary_tree
>
> ##### Difficulty:
>
> #coding_problem #difficulty_easy
>
> ##### Additional tags:
>
> #leetcode
>
> ##### Revisions:
>
> N/A

##### Related topics:

##### Links:

- [Link to problem](https://leetcode.com/problems/invert-binary-tree/)

---

### Problem

Given the `root` of a binary tree, invert the tree, and return _its root_.

#### Examples

**Example 1:**

![](https://assets.leetcode.com/uploads/2021/03/14/invert1-tree.jpg)

```
**Input:** root = [4,2,7,1,3,6,9]
**Output:** [4,7,2,9,6,3,1]
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2021/03/14/invert2-tree.jpg)

```
**Input:** root = [2,1,3]
**Output:** [2,3,1]
```

**Example 3:**

```
**Input:** root = []
**Output:** []
```

#### Constraints

- The number of nodes in the tree is in the range `[0, 100]`.
- `-100 <= Node.val <= 100`

### Thoughts

> [!summary]
> This is a #DFS like recursion problem.

Very simple, think of base cases:

- the node is void, skip.
  And the flow is following

- Catch base case
- Invert sub-trees first
- invert left and right node

### Solution

```cpp
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left),
 * right(right) {}
 * };
 */
class Solution {
  void invert(TreeNode *root) {
    if (!root) {
      return;
    }

    invert(root->left);
    invert(root->right);

    TreeNode *tmp = root->left;
    root->left = root->right;
    root->right = tmp;
  }

public:
  TreeNode *invertTree(TreeNode *root) {
    // Using DFS-like Recursion
    invert(root);
    return root;
  }
};
```