Preamble variable postorder traversal, in order traversal

Before seeing the data structure of the book.
Assume that node is the root node of the tree, variable, then simply print it.

• Preorder traversal

Node{
    Node left;
    Node right;
    Object val;
}

public void travel(Node root){
    if(root!=null){
        System.out.println(root.val)
        travel(root.left);
        travel(root.right);
    }
}

• Preorder

Node{
    Node left;
    Node right;
    Object val;
}

public void travel(Node root){
    if(root!=null){
        travel(root.left);
        System.out.println(root.val);
        travel(root.right);
    }
}

• Postorder

Node{
    Node left;
    Node right;
    Object val;
}

public void travel(Node root){
    if(root!=null){
        travel(root.left);
        travel(root.right);
        System.out.println(root.val);
    }
}

A closer look at the top is to the preamble, in that the intermediate sequence, the sequence is later. Such a thought, instantly clear.