Reorder List

Given a singly linked list L: L0 → L1 → … → Ln-1 → Ln

reorder it to: L0 → Ln → L1 → Ln-1 → L2 → Ln-2 → …

链表的题目非常容易出错，要仔细些。尤其要清楚其中的指针是怎么变化的

java

/** * Definition for ListNode. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int val) { *         this.val = val; *         this.next = null; *     } * } */public class Solution {    /*     * @param head: The head of linked list.     * @return: nothing     */    public void reorderList(ListNode head) {        // write your code here        if (head == null || head.next == null) {            return;        }        ListNode slow = head;        ListNode fast = head.next;        ListNode mid = null;        while (fast != null && fast.next != null) {            slow = slow.next;            fast = fast.next.next;        }        mid = slow;        ListNode tail = reverse(mid.next);        mid.next = null;        merge(head, tail);    }    private ListNode reverse(ListNode head) {        if (head == null) {            return null;        }        ListNode curr = head;        ListNode prev = null;        while (curr != null) {            ListNode temp = curr.next;            curr.next = prev;            prev = curr;            curr = temp;        }        return prev;    }    private void merge(ListNode head, ListNode tail) {        if (head == null || tail == null) {            return;        }        int index = 0;        ListNode dummy = new ListNode(0);        while (head != null && tail != null) {            dummy.next = head;            head = head.next;            dummy = dummy.next;            dummy.next = tail;            tail = tail.next;            dummy = dummy.next;        }        if (head != null) {            dummy.next = head;        } else {            dummy.next = tail;        }    }}

python

"""Definition of ListNodeclass ListNode(object):    def __init__(self, val, next=None):        self.val = val        self.next = next"""class Solution:    """    @param: head: The head of linked list.    @return: nothing    """    def reorderList(self, head):        # write your code here        if head is None or head.next is None:            return        mid = self.findMid(head)        tail = self.reverse(mid.next)        mid.next = None        self.merge(head, tail)            def findMid(self, head):        slow = head        fast = head.next        while fast is not None and fast.next is not None:            slow = slow.next            fast = fast.next.next        return slow        def reverse(self, head):        if head is None:            return head        curr = head        prev = None        while curr is not None:            temp = curr.next            curr.next = prev            prev = curr            curr = temp        return prev            def merge(self, head, tail):        if head is None or tail is None:            return        flag = True        dummy = ListNode(-1)        while head is not None and tail is not None:            if flag:                dummy.next = head                head = head.next            else:                dummy.next = tail                tail = tail.next            flag = not flag            dummy = dummy.next        if head is not None:            dummy.next = head        else:            dummy.next = tail