LeetCode刷题笔录 Reorder List

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}.

在ctci的链表那节有个类似的题。这里这道题要多一步反转链表而已。

1.把链表从中间分成前后两部分。如果链表长度是奇数，那么前面的链表长度多一。

2.把后半部分链表反转

3.再把两个链表合并

没有用到extra space，时间复杂度O(n)

public class Solution {    public void reorderList(ListNode head) {        if(head == null)            return;        //divide the list into two lists        ListNode fast = head;        ListNode slow = head;        while(fast.next != null){            fast = fast.next;            if(fast.next != null){                fast = fast.next;            }            else{                break;            }            slow = slow.next;        }                //reverse the second part        ListNode secondHead = reverseList(slow.next);        slow.next = null;                //merge the two lists        merge(head, secondHead);    }        public ListNode reverseList(ListNode node){        ListNode p = node;        if(p == null || p.next == null)            return p;        ListNode q = p.next;        p.next = null;        while(q != null){            ListNode r = q.next;            q.next = p;            p = q;            q = r;        }                return p;    }        public void merge(ListNode node1, ListNode node2){        ListNode p = node1;        ListNode q = node2;        while( p != null && q != null){            ListNode r = p.next;            ListNode m = q.next;            p.next = q;            q.next = r;            p = r;            q = m;        }    }}