链表的排序

题目描述

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

由于限制了时间复杂度，所以选择使用归并排序

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode sortList(ListNode head) {
        if(head == null || head.next==null)
            return head;
        
        ListNode slow = head;
        ListNode fast = head;
        while(fast.next!=null && fast.next.next!=null){
            slow = slow.next;
            fast = fast.next.next;
        }
        
        fast = slow.next;
        slow.next = null;
        slow =  sortList(head);
        fast = sortList(fast);
        head = mergeSort(slow,fast);  
        return head;
    }
    
    public ListNode mergeSort(ListNode head1,ListNode head2){
        if(head1==null)
            return head2;
        if(head2 ==null)
            return head1;
        ListNode head=null;
        
        if(head1.val>head2.val){
            head = head2;
            head.next = mergeSort(head1,head2.next);
        }else{
            head = head1;
            head.next = mergeSort(head1.next,head2);
        }
        
        return head;
    }
}