GeeksforGeeks App
Open App
Browser
Continue

# Java Program For Flattening A Multilevel Linked List

Given a linked list where in addition to the next pointer, each node has a child pointer, which may or may not point to a separate list. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure, as shown in the below figure. You are given the head of the first level of the list. Flatten the list so that all the nodes appear in a single-level linked list. You need to flatten the list in a way that all nodes at the first level should come first, then nodes of the second level, and so on.
Each node is a C struct with the following definition.

## Java

 static class List{    public int data;    public List next;    public List child;};// This code is contributed by pratham76

The above list should be converted to 10->5->12->7->11->4->20->13->17->6->2->16->9->8->3->19->15

The problem clearly says that we need to flatten level by level. The idea of a solution is, we start from the first level, process all nodes one by one, if a node has a child, then we append the child at the end of the list, otherwise, we don’t do anything. After the first level is processed, all next-level nodes will be appended after the first level. The same process is followed for the appended nodes.

1) Take the "cur" pointer, which will point to the head
of the first level of the list
2) Take the "tail" pointer, which will point to the end of
the first level of the list
3) Repeat the below procedure while "curr" is not NULL.
I) If the current node has a child then
a) Append this new child list to the "tail"
tail->next = cur->child
b) Find the last node of the new child list and update
the "tail"
tmp = cur->child;
while (tmp->next != NULL)
tmp = tmp->next;
tail = tmp;
II) Move to the next node. i.e. cur = cur->next

Following is the implementation of the above algorithm.