Invert Binary Tree

Invert a binary tree.

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

to

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

From 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. 关于这个故事的连接 [https://twitter.com/mxcl/status/608682016205344768?lang=en]

递归解法

public TreeNode invertTree(TreeNode root) {
    if (root == null) {
        return null;
    }
    TreeNode left = root.left;
    TreeNode right = root.right;
    root.left = invertTree(right);
    root.right = invertTree(left);
    return root;
}