/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root==null) return null;
        exchange(root);
        return root;
    }

    public void exchange(TreeNode node){
        if(node.left==null&&node.right==null)
            return;
        else{
            if(node.left==null&&node.right!=null){
                node.left=node.right;
                node.right=null;
                exchange(node.left);
                return;
            }else if(node.right==null&&node.left!=null){
                node.right=node.left;
                node.left=null;
                exchange(node.right);
            }else{
                TreeNode temp = node.left;
                node.left = node.right;
                node.right = temp;
                exchange(node.left);
                exchange(node.right);
            }
        }
    }
}