26.翻转二叉树(题号：226)

//方法一：先序遍历
public TreeNode invertTree(TreeNode root) {
    if ( root == null ) return null;
    TreeNode temp = root.left;
    root.left = root.right;
    root.right = temp;
    invertTree( root.left );
    invertTree( root.right );
    return root;
}
//方法二：后序遍历
public TreeNode invertTree(TreeNode root) {
    if ( root == null ) return null;
    TreeNode left = invertTree(root.left);
    TreeNode right = invertTree(root.right);
    root.left = right;
    root.right = left;
    return root;
}