226. Invert Binary Tree

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latest version

T : O(n)

S: O(n)

思路 左邊接右邊dfs, 右邊接左邊 dfs

```java
class Solution {
    public TreeNode invertTree(TreeNode root) {
        return dfs(root);
    }
    private TreeNode dfs(TreeNode node) {
        if (node == null) {
            return null;
        }
        TreeNode root = new TreeNode(node.val);
        root.left = dfs(node.right);
        root.right = dfs(node.left);
        return root;
    }
}
```

左右對調就是了, like swapping

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) { //termainator
            return root;
        }
        // like swap - process
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        
        invertTree(root.left); // drill dwon
        invertTree(root.right); // drill dwon
        return root;
    }
}

or like this

    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }

        TreeNode temp = root.left;
        root.left = invertTree(root.right);
        root.right = invertTree(temp);
        return root;
    }