148. Sort List

148. Sort List

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

利用mergesort 

merge 操作只能合并2个有序的子序列

所以利用递归对每个子序列进行排序，排序后merge。

class Solution {
    public ListNode sortList(ListNode head) {
        if(head==null || head.next==null) return head;
        
        //1 split 
        ListNode pre=null,slower =head,faster=head;
        while(faster!=null && faster.next!= null){
            pre = slower;
            slower = slower.next;
            faster = faster.next.next;
        }
        pre.next = null;
        //2 sort 
        ListNode l1 = sortList(head);
        ListNode l2 = sortList(slower);
        //3 merge
        return Merge(l1,l2);
        
    }
    
    public ListNode Merge(ListNode l1,ListNode l2){
        ListNode l ,p;
        l = new ListNode(0);
        p=l;
        
        while(l1 != null&& l2 != null){
            if(l1.val<l2.val){
                p.next = l1;
                l1 = l1.next;
            }
            else{
                p.next = l2;
                l2 = l2.next;
            }
            p = p.next;
        }
        if(l1==null) p.next = l2;
        if(l2==null) p.next = l1;
        return l.next;
    }
}