Invert a binary tree.

4   /   \  2     7 / \   / \1   3 6   9

to

4   /   \  7     2 / \   / \9   6 3   1

1 /** 2  * Definition for a binary tree node. 3  * public class TreeNode { 4  *     public int val; 5  *     public TreeNode left; 6  *     public TreeNode right; 7  *     public TreeNode(int x) { val = x; } 8  * } 9  */10 public class Solution {11     public TreeNode InvertTree(TreeNode root) {12         if (root == null) return root;13         14         var right = root.right;15         root.right = InvertTree(root.left);16         root.left = InvertTree(right);17         18         return root;19     }20 }