Leetcode专题之 - 链表

下面是一些常考链表的知识：

1. 反转链表
2. 寻找链表的中点
3. 两个有序链表排序
4. 链表排序：merge sort Given the head of a linked list, return the list after sorting it in ascending order.

Example 1:

Input: head = [4,2,1,3]
Output: [1,2,3,4]

Example 2:

Input: head = [-1,5,3,4,0]
Output: [-1,0,3,4,5]

Example 3:

Input: head = []
Output: []

使用归并排序

class Solution {
    public ListNode sortList(ListNode head) {
        return sortList(head, null);
    }

    public ListNode sortList(ListNode head, ListNode tail) {
        if(head == null) return head;
        if(head.next == tail) {
            head.next = null;
            return head;
        }
        ListNode slow = head;
        ListNode fast = head;
        while(fast != tail) {
            slow = slow.next;
            fast = fast.next;
            if(fast != tail) {
                fast = fast.next;
            }
        }
        ListNode mid = slow;
        ListNode list1 = sortList(head, mid);
        ListNode list2 = sortList(mid, tail);
        ListNode sorted = merge(list1, list2);
        return sorted;

    }

    public ListNode merge(ListNode head1, ListNode head2) {
        ListNode dummy = new ListNode(0);
        ListNode l1 = head1;
        ListNode l2 = head2;
        ListNode curr = dummy;
        while(l1 != null && l2 != null) {
            if(l1.val < l2.val) {
                curr.next = l1;
                l1 = l1.next;
            } else {
                curr.next = l2;
                l2 = l2.next;
            }
            curr = curr.next;
        }
        if(l1 != null) {
            curr.next = l1;
        } else if(l2 != null) {
            curr.next = l2;
        }
        return dummy.next;
    }
}