leetcode-sort-list

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题目大意

　　https://leetcode.com/problems/sort-list/

　　单链表排序，要求时间复杂度O(n*logn)，空间复杂度：O(1)

题目分析

　　先利用快慢指针找到链表中点，然后每段递归调用，最后merge两个链表

代码

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public class Solution {
    
    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) { // 这里注意head.next == null的判断
            return head;
        }
        ListNode slow = head;
        ListNode fast = head;
        ListNode pre = null;
        while (fast != null) {
            fast = fast.next;
            if (fast != null) {
                fast = fast.next;
            }
            pre = slow;
            slow = slow.next;
        }
    
        ListNode p1 = sortList(slow);
        pre.next = null;
        ListNode p2 = sortList(head);
        // merge two sorted linked list
        ListNode newHead = new ListNode(-1);
        ListNode cur = newHead;
        while (p1 != null || p2 != null) {
            if (p1 == null) {
                cur.next = p2;
                break;
            }
            if (p2 == null) {
                cur.next = p1;
                break;
            }
            if (p1.val < p2.val) {
                cur.next = p1;
                p1 = p1.next;
            } else {
                cur.next = p2;
                p2 = p2.next;
            }
            cur = cur.next;
        }
        return newHead.next;
        
    }
}