归并排序Merge Sort LinkedList in 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();
}

}
}