复习一下二叉树的前中后序遍历
就我所知道的，有递归和非递归两种方法。

//递归版前序遍历
public static void TreePreOrder(TreeNode node){
        System.out.println(node.val);
        TreeNode leftTree = node.left;
        if (leftTree != null){
            TreePreOrder(leftTree);
        }
        TreeNode rightTree = node.right;
        if(rightTree != null){
            TreePreOrder(rightTree);
        }
}

//递归版中序遍历
public static void TreeInOrder(TreeNode node){
        if(node == null) return;

        TreeInOrder(node.left);
        System.out.println(node.val);
        TreeInOrder(node.right);
}

//递归版后序遍历
public static void TreePostOrder(TreeNode node){
        if(node == null) return;

        TreePostOrder(node.left);
        TreePostOrder(node.right);
        System.out.println(node.val);
}