题目：Invert a binary tree.

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

to

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

下面是我给出的算法：

/** * 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;
    }
}