Reorder List

题目：

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

思路：

Reorder List, reverse linkedList I & II, Sort List,还有好几道leetcode的关于链表的题，其实具体考察的就是三个操作。

LinkedList 的 Partition，Reverse 和 Merge

我当时是认真做了一下LinkedList 的merge sort，就对这些操作比较熟练了。

    public void reorderList(ListNode head) {
        ListNode l2 = partition(head);
        ListNode newL2 = reverse(l2);
        merge(head, newL2);
    }
    
    private ListNode partition (ListNode head) {
        ListNode left = head;
        ListNode right = head;
        ListNode pre = new ListNode(0);
        while (right != null) {
            pre = left;
            left = left.next;
            right = right.next;
            if (right != null) {
                right = right.next;
            }
        }
        pre.next = null;
        return left;
    }
    
    private ListNode reverse(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode pre = null;
        ListNode cur = head;
        ListNode temp = new ListNode(0);
        while(cur != null) {
            temp = cur.next;
            cur.next = pre;
            pre = cur;
            cur = temp;
        }
        return pre;
    }
    private void merge(ListNode l1, ListNode l2) {
        ListNode temp = new ListNode(0);
        while (l1 != null && l2 != null) {
            temp = l1.next;
            l1.next = l2;
            l1 = temp;
            if (l1 == null) {
                return;
            }
            temp = l2.next;
            l2.next = l1;
            l2 = temp;
            if (l2 == null) {
                return;
            }
        }
    }