Question

I have a singly linked list. Apart from the normal “Next” pointer, there is one more pointer(random ptr) in each node which points to some random node of the list. How to create a clone of such a list? (In less than O(n^2)).

Any suggestion or solution using Java?

Answer 1

Here is an answer in O(n) time and O(1) space.
(Solutions with a hashtable or association map need O(n) space).
shg’s link is also a solution in O(n) time and O(1) space.

• traverse the list to count the number of cells, n.
• Create an array t of size n, each cell consisting of two pointers a and b. At the end of the algorithm, this will be the copy. But it isn’t for now.
• traverse the original list. For the k^th cell c of the original list:
  • let t[k].a be a pointer to c
  • let c.next be a pointer to t[k] (the original list is temporarily destroyed, we will restore it later.). We can now follow pointers back and forth between the original list and t.
• traverse the original list, and for each cell c, let c.next.b be a pointer to c.random.next. (c.next is a cell in t, so is c.random.next). This way, the b fields of the cells in t are a copy of the structure of the random pointers in the original list.
• restore the original list: for each k, let t[k].a.next point to t[k+1].a.next
• make t a linked list: for each k, let t[k].a point to t[k+1].

As opposed to shg’s link, this solution has the drawback of requiring a continuous block of size n in memory.