Question

I was asked in a interview about the difference between node structures of Doubly linked list and binary Tree.

Doubly Linked List Struct

typedef struct
{
int data;
struct node * next;
struct node * prev;
}node;    

Binary Tree Struct

typedef struct
{
int data;
struct node * left;
struct node * right;
}node;  

1. In Doubly linked list, we use pointers for traversing back and front in a linearly arranged list.
2. But where as left & right pointers are used to access the left & right nodes.

I don’t find any difference in node structure except the way they are used. Could you please give me some differences???

Answer 1

I think you have answered your own questions. Other than the obvious differences in names of the pointers (next/prev and left/right), the differences are:

• In a doubly linked list, if n.next links to m then m.prev links to n. This is not the case in a binary tree.
• In a doubly linked list, a node can have at most two links to it. In a binary tree, each node has at most one link to it.
• In a doubly linked list, it is possible to follows links and end up at the node where you started. This is not possible in a binary tree.

If the doubly linked list is also cyclic, the following also holds:

• In a cyclic doubly linked list with one node, n.next links to n and n.prev also links to n. This is not possible in a binary tree: n.left and n.right do not link to the same node.
• In a binary tree with one node, n.next and n.prev could point to no node (i.e., the tree is just a leaf node) but in a cyclic doubly linked list with one node both links always have a value (albeit to the same node).
• In a binary tree it is optional to have a value for either n.left or n.right. If the binary tree is not balanced, it could be that all nodes have a value for n.right but not for n.left. This is not the case for a cyclic doubly linked list where all pointers have a value.

In terms of structure and content the nodes are the same but the differences are in how the pointers are used and what their values can be.