【leetcode 148 Sort List】

Sort a linked list in O(n log n) time using constant space complexity.

既然要求复杂度为O(nlogn)，那么要么是快速排序要么是归并排序，这里采用归并排序，需要注意mid分割点的寻找

采用快慢指针得到mid节点，然后分为两段进行排序并合并，递归进行。代码如下：

 public ListNode sortList(ListNode head) {
         if(head==null||head.next==null)
return head;
 ListNode slow = head;
 ListNode fast = head;
 ListNode result = head;
 ListNode pre = head;
 while(fast!=null&&fast.next!=null){
 pre = slow;
 slow = slow.next;
 fast = fast.next.next;
 }
 pre.next = null;
 ListNode h1 = head;
 ListNode h2 = slow;
 //slow.next = null;
 ListNode s1 = sortList(h1);
 ListNode s2 = sortList(h2);
 result = merge(s1,s2);  
 return result;
    }
    
     public static ListNode merge(ListNode h1, ListNode h2) {
if(h1==null)
return h2;
if(h2==null)
return h1;
ListNode head = null;
ListNode p = h1;
ListNode q = h2;
if(h1.val<=h2.val){
head = h1;
p = p.next;
}
else {
head = h2;
q = q.next;
}
head.next = null;
ListNode k = head;

 while (p!=null && q!=null) {
  if (p.val<q.val) {
  k.next = p;
  p = p.next;
  k = k.next;
  k.next = null;
  } else {
  k.next = q;
  q = q.next;
  k = k.next;
  k.next = null;
  }
 }


 if (p!=null) {
 k.next = p;
 }


 if (q!=null) {
 k.next = q;
 }
 return head;
}