链表排序 归并的链表排序

79.
1.编写实现链表排序的一种算法。说明为什么你会选择用这样的方法？
ANSWER
For linked list sorting, usually mergesort is the best choice. Pros: O(1) auxilary space, compared to array merge sort. No node creation, just pointer operations. 
Node * linkedListMergeSort(Node * pHead) {
  int len = getLen(pHead);
  return mergeSort(pHead, len);
}

Node * mergeSort(Node * p, int len) {
  if (len == 1) { p->next = NULL; return p; }
  Node * pmid = p;
  for (int i=0; i<len/2; i++) {
    pmid = pmid->next;
  }
  Node * p1 = mergeSort(p, len/2);
  Node * p2 = mergeSort(pmid, len - len/2);
  return merge(p1, p2);
}
Node * merge(Node * p1, Node * p2) {
  Node * p = NULL, * ph = NULL;
  while (p1!=NULL && p2!=NULL) {
    if (p1->data<p2->data) {
      if (ph == NULL) {ph = p = p1;}
      else { p->next = p1; p1 = p1->next; p = p->next;}
    } else {
      if (ph == NULL) {ph = p = p2;}
      else { p->next = p2; p2 = p2->next; p = p->next;}
    }
  }
  p->next = (p1==NULL) ? p2 : p1;
  return ph;
}