Invert a binary tree.

不用多说，递归

/**
 * 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) {
        if(root == null)
            return null;

        TreeNode left = invertTree(root.left);
        TreeNode right = invertTree(root.right);
        TreeNode temp = left;
        root.left = right;
        root.right = temp;

        return root;
    }
}