【LeetCode】#148排序連結串列(Sort List)

題目描述

在 O(n log n) 時間複雜度和常數級空間複雜度下，對連結串列進行排序。

示例

示例 1:

輸入: 4->2->1->3
輸出: 1->2->3->4

示例 2:

輸入: -1->5->3->4->0
輸出: -1->0->3->4->5

Description

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

Example

Example 1:

Input: 4->2->1->3
Output: 1->2->3->4

Example 2:

Input: -1->5->3->4->0
Output: -1->0->3->4->5

解法

class Solution {
    public ListNode sortList(ListNode head) {
        return head == null ? null : mergeSort(head);
    }
    
    private ListNode mergeSort(ListNode head){
        if(head.next==null){
            return head;
        }
        ListNode p = head, q = head, pre = null;
        while(q!=null && q.next!=null){
            pre = p;
            p = p.next;
            q = q.next.next;
        }
        pre.next = null;
        ListNode l = mergeSort(head);
        ListNode r = mergeSort(p);
        return merge(l, r);
    }
    
    ListNode merge(ListNode l, ListNode r){
        ListNode dummyHead = new ListNode(0);
        ListNode cur = dummyHead;
        while(l!=null && r!=null){
            if(l.val<=r.val){
                cur.next = l;
                cur = cur.next;
                l = l.next;
            }else{
                cur.next = r;
                cur = cur.next;
                r = r.next;
            }
            
        }
        if(l!=null){
           cur.next = l;
        }
        if(r!=null){
            cur.next = r;
        }
        return dummyHead.next;
    }
}