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;
        }
    }
}