http://www.programcreek.com/2012/11/leetcode-solution-merge-sort-linkedlist-in-java/

MergeSort.java

package algorithm.sort;
 
class ListNode {
	int val;
	ListNode next;
 
	ListNode(int x) {
		val = x;
		next = null;
	}
}
 
public class SortLinkedList {
 
	// merge sort
	public static ListNode mergeSortList(ListNode head) {
 
		if (head == null || head.next == null)
			return head;
 
		// count total number of elements
		int count = 0;
		ListNode p = head;
		while (p != null) {
			count++;
			p = p.next;
		}
 
		// break up to two list
		int middle = count / 2;
 
		ListNode l = head, r = null;
		ListNode p2 = head;
		int countHalf = 0;
		while (p2 != null) {
			countHalf++;
			ListNode next = p2.next;
 
			if (countHalf == middle) {
				p2.next = null;
				r = next;
			}
			p2 = next;
		}
 
		// now we have two parts l and r, recursively sort them
		ListNode h1 = mergeSortList(l);
		ListNode h2 = mergeSortList(r);
 
		// merge together
		ListNode merged = merge(h1, h2);
 
		return merged;
	}
 
	public static ListNode merge(ListNode l, ListNode r) {
		ListNode p1 = l;
		ListNode p2 = r;
 
		ListNode fakeHead = new ListNode(100);
		ListNode pNew = fakeHead;
 
		while (p1 != null || p2 != null) {
 
			if (p1 == null) {
				pNew.next = new ListNode(p2.val);
				p2 = p2.next;
				pNew = pNew.next;
			} else if (p2 == null) {
				pNew.next = new ListNode(p1.val);
				p1 = p1.next;
				pNew = pNew.next;
			} else {
				if (p1.val < p2.val) {
					// if(fakeHead)
					pNew.next = new ListNode(p1.val);
					p1 = p1.next;
					pNew = pNew.next;
				} else if (p1.val == p2.val) {
					pNew.next = new ListNode(p1.val);
					pNew.next.next = new ListNode(p1.val);
					pNew = pNew.next.next;
					p1 = p1.next;
					p2 = p2.next;
 
				} else {
					pNew.next = new ListNode(p2.val);
					p2 = p2.next;
					pNew = pNew.next;
				}
			}
		}
 
		// printList(fakeHead.next);
		return fakeHead.next;
	}
 
	public static void main(String[] args) {
		ListNode n1 = new ListNode(2);
		ListNode n2 = new ListNode(3);
		ListNode n3 = new ListNode(4);
 
		ListNode n4 = new ListNode(3);
		ListNode n5 = new ListNode(4);
		ListNode n6 = new ListNode(5);
 
		n1.next = n2;
		n2.next = n3;
		n3.next = n4;
		n4.next = n5;
		n5.next = n6;
 
		n1 = mergeSortList(n1);
 
		printList(n1);
	}
 
	public static void printList(ListNode x) {
		if(x != null){
			System.out.print(x.val + " ");
			while (x.next != null) {
				System.out.print(x.next.val + " ");
				x = x.next;
			}
			System.out.println();
		}
 
	}
}