LeetCode 148 Sort List

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

思路：要想时间复杂度达到O(n log n)
，那么有两种。一种是合并排序，还有一种是高速排序，而要想空间复杂度为常数。那么仅仅能使用递归，本人使用的是递归的合并排序。代码例如以下：

/**
* Definition for singly-linked list.
* class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) {
*         val = x;
*         next = null;
*     }
* }
*/
public class Solution {
private ListNode merge(ListNode l1, ListNode l2) {
if(l1==null&&l2==null) return null;
ListNode head=new ListNode (-1);
ListNode pNode=head;
while(l1!=null||l2!=null){
if(l1==null) {
pNode.next=l2;
return head.next;
}
if(l2==null){
pNode.next=l1;
return head.next;
}
if(l1.val>l2.val){
pNode.next=l2;
l2=l2.next;
}else {
pNode.next=l1;
l1=l1.next;
}
pNode=pNode.next;
}
return head.next;
}
private ListNode mergeSort(ListNode head){
if(head==null||head.next==null)   //just one element
return head;
ListNode p=head, q=head, pre=null;
while(q!=null&&q.next!=null){
q=q.next.next;
pre=p;
p=p.next;  //divide into two parts
}
pre.next=null;
ListNode lhalf=mergeSort(head);
ListNode rhalf=mergeSort(p);  //recursive
return merge(lhalf, rhalf);   //merge
}
public ListNode sortList(ListNode head) {
return mergeSort( head);
}
}