Leetcode430-flattenaMultilevelDoublyLinkedList

Description

You are given a doubly linked list which in addition to the next and previous pointers, it could have a child pointer, which may or may not point to a separate doubly linked 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 example below.

Flatten the list so that all the nodes appear in a single-level, doubly linked list. You are given the head of the first level of the list.

Example

Input:
 1---2---3---4---5---6--NULL
         |
         7---8---9---10--NULL
             |
             11--12--NULL

Output:
1-2-3-7-8-11-12-9-10-4-5-6-NULL

Explanation for the above example:

Given the following multilevel doubly linked list:

We should return the following flattened doubly linked list:

Solution

Method 1: Iteration

/*
// Definition for a Node.
class Node {
    public int val;
    public Node prev;
    public Node next;
    public Node child;

    public Node() {}

    public Node(int _val,Node _prev,Node _next,Node _child) {
        val = _val;
        prev = _prev;
        next = _next;
        child = _child;
    }
};
*/
class Solution {
    public Node flatten(Node head) {
        if (head == null) return head;
        Node p = head;
        
        while( p != null){
            if (p.child == null){
                p = p.next;
                continue;
            }
            else{
                Node childTail = p.child;
                while(childTail.next != null) childTail = childTail.next;
                childTail.next = p.next;
                if (p.next != null) p.next.prev = childTail;
                
                p.next = p.child;
                p.child.prev = p;
                p.child = null;
                
                p = p.next;
            }
        }
        return head;
    }
}

Mathod 2: Recursion

/*
// Definition for a Node.
class Node {
    public int val;
    public Node prev;
    public Node next;
    public Node child;

    public Node() {}

    public Node(int _val,Node _prev,Node _next,Node _child) {
        val = _val;
        prev = _prev;
        next = _next;
        child = _child;
    }
};
*/
class Solution {
    public Node flatten(Node head) {
        if (head == null) return null;      
        helper(head);
        return head;
    }   
    private Node helper(Node cur){
        if (cur == null) return cur;
        if (cur.child == null){
            if (cur.next == null) return cur;
            return helper(cur.next);
        }
        else{
            Node child = cur.child;
            cur.child = null;
            Node next = cur.next;
            Node childTail = helper(child);
            cur.next = child;
            child.prev = cur;
            if (next != null){
                childTail.next = next;
                next.prev = childTail;
                return helper(next);
            }
            return childTail;
        }
    }
}

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