Java: Construct a linked list from 2D matrix

Difficulty Level: HARD

Problem Statement:

Given a matrix. Convert it into a linked list matrix such that each node is connected to its next right and down node.

Input:

2D Matrix: 
1 5 2 6 
9 3 4 8 
2 3 6 9 
7 8 5 1 

Output:

1->5->2->6->NULL
9->3->4->8->NULL 
2->3->6->9->NULL 
7->8->5->1->NULL