CODE

/**
* Definition for a binary tree node.
* public class TreeNode {
*     public int val;
*     public TreeNode left;
*     public TreeNode right;
*     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
*         this.val = val;
*         this.left = left;
*         this.right = right;
*     }
* }
*/

public class Solution {
  public TreeNode InvertTree(TreeNode root) {

    if (root == null)
      return root;

    TreeNode temp = root.left;
    root.left = root.right;
    root.right = temp;

    InvertTree(root.left);
    InvertTree(root.right);
    return root;
  }
}