Time Complexity=O(n), Space complexity=O(1)
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode() {}
* ListNode(int val) { this.val = val; }
* ListNode(int val, ListNode next) { this.val = val; this.next = next; }
* }
*/
class Solution {
public void reorderList(ListNode head) {
if (head == null || head.next == null) {
return;
}
ListNode first = head;
ListNode slow = head;
ListNode fast = head;
while (fast.next != null && fast.next.next != null) {
slow = slow.next;
fast = fast.next.next;
}
ListNode second = slow;
second = reverse(second);
while (first != null) {
ListNode temp = first.next;
first.next = second;
first = temp;
temp = second.next;
second.next = first;
second = temp;
}
}
private ListNode reverse(ListNode head) {
if (head == null || head.next == null) {
return head;
}
ListNode newHead = reverse(head.next);
head.next.next = head;
head.next = null;
return newHead;
}
}