Leetcode#148 Sort List

 1 void mergeList(ListNode *a, ListNode *b, ListNode *&h, ListNode *&t) {
 2   h = t = NULL;
 3   while (a && b) {
 4     ListNode *c = NULL;
 5     if (a->val <= b->val) {
 6       c = a;
 7       a = a->next;
 8     }
 9     else {
10       c = b;
11       b = b->next;
12     }
13     if (!h)
14       h = t = c;
15     else {
16       t->next = c;
17       t = t->next;
18     }
19   }
20   while (a) {
21     t->next = a;
22     t = t->next;
23     a = a->next;
24   }
25   while (b) {
26     t->next = b;
27     t = t->next;
28     b = b->next;
29   }
30 }
31     
32 ListNode *sortList(ListNode *head) {
33   ListNode *prev = NULL;
34   ListNode *h1 = NULL;
35   ListNode *h2 = NULL;
36   ListNode *t1 = NULL;
37   ListNode *t2 = NULL;
38   ListNode *node = NULL;
39   int len = 0;
40         
41   node = head;
42   while (node && (++len))
43     node = node->next;
44             
45   for (int l = 1; l < len; l <<= 1) {
46     prev = NULL;
47     h1 = NULL;
48     h2 = NULL;
49     t1 = NULL;
50     t2 = NULL;
51     node = head;
52             
53     while (node) {
54       h1 = t1 = node;
55       for (int i = 0; node && i < l; i++) {
56         t1 = node;
57         node = node->next;
58       }
59       if (t1)
60         t1->next = NULL;
61       h2 = t2 = node;
62       for (int i = 0; node && i < l; i++) {
63         t2 = node;
64         node = node->next;
65       }
66       if (t2)
67         t2->next = NULL;
68                 
69       ListNode *h, *t;
70       if (h2)
71         mergeList(h1, h2, h, t);
72       else {
73         h = h1;
74         t = t1;
75       }
76       if (!prev)
77         head = h;
78       else
79         prev->next = h;
80       t->next = node;
81       prev = t;
82     }
83   }
84         
85   return head;
86 }