Problem Sort List

 1     public ListNode sortList(ListNode head) {
 2         ListNode tail , p, q, e;
 3         int  insize, merges, psize, qsize, i;
 4         if (head == null) return null; 
 5         
 6 
 7         insize = 1;
 8         while (true) {
 9             p = head;
10             head = null;
11             tail  = null;
12 
13             merges = 0;
14 
15             while (p != null) {
16 
17                 merges++;
18                 q = p;
19                 psize = 0;
20                 for (i = 0; i < insize; i++) {
21                     psize++;
22                     q = q.next;
23                     if (q == null) break;
24                 }
25 
26                 qsize = insize;
27                 while (psize > 0 || (qsize > 0 && q != null)) {
28                     if (psize == 0) {
29                         e = q; q = q.next; qsize--;
30                     } else if(qsize == 0 ||  q == null) {
31                         e = p; p = p.next; psize--;
32                     } else if ((p.val - q.val) <= 0) {
33                         e = p; p = p.next; psize--;
34                     } else {
35                         e = q; q = q.next; qsize--;
36                     }
37 
38                     if (tail != null) {
39                         tail.next = e;
40                     } else {
41                         head = e;
42                     }
43 
44                     tail = e;
45                 }
46 
47                 p = q;
48             }
49 
50             tail.next = null;
51 
52             if (merges <= 1) return head;
53 
54             insize *= 2; 
55         }
56     }