LeetCode Solutions : 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 itto {1,4,2,3}.
Considering the following steps:
* 1. split such list into two list, first and second, according to slow and fast point
* 2. reverse the second list
* 3. insert the second list into the first list
coding solution:
/**
* Definition for singly-linked list.
* class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*
*/
public class Solution {
public void reorderList(ListNode head) {
if(head!=null&&head.next!=null){
ListNode low=head;//1. split such list into two list, first and second, according to slow and fast point
ListNode fast=head;
while(fast.next!=null&&fast.next.next!=null){
low=low.next;
fast=fast.next.next;
}
ListNode first=head;
ListNode second=low.next;
low.next=null; second=reverse(second);//2. reverse the second list
while(second!=null){//3. insert the second list into the first list
ListNode p1=first.next;
ListNode p2=second.next;
first.next=second;
second.next=p1;
first=p1;
second=p2;
} }
}
private ListNode reverse(ListNode head){
if(head==null||head.next==null)
return head;
ListNode pre=head;
ListNode cur=head.next;
while(cur!=null){
ListNode nextNode=cur.next;
cur.next=pre;
pre=cur;
cur=nextNode;
}
head.next=null;
return pre;
}
}
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