題目：Invert a binary tree.

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

to

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

下面是我給出的演算法：

/**
 * 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;
        }
        if(root.left!=null){
            invertTree(root.left);
        }
        if(root.right!=null){
            invertTree(root.right);
        }
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        return
 
 root;
    }
}