很简单的一题，难怪某大牛没写出来被google喷了。。
This problem was inspired by this original tweet by Max Howell:Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.

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