143. Reorder List

//O(n) time and O(1) space
class Solution {
    public void reorderList(ListNode head) {
        if(head==null || head.next==null) return;
        ListNode slow = head, fast = head;
        while(fast!=null && fast.next!=null){
            slow = slow.next;
            fast = fast.next.next;
        }
        if(fast!=null) slow = slow.next;
        ListNode l1 = head;
        ListNode l2 = reverse(slow);
        ListNode h1 = l1, h2 = l2;
        while(l1!=null && l2!=null && !l1.equals(h2)){
            ListNode fnext = l1.next, snext = l2.next;
            l1.next = l2;
            l2.next = fnext;
            l1 = fnext;
            l2 = snext;
        }
        l1.next = l2;
    }
    
    public ListNode reverse(ListNode head){
        if(head==null || head.next==null) return head;
        ListNode cur = head, prev = null;
        while(cur!=null){
            ListNode next = cur.next;
            cur.next = prev;
            prev = cur;
            cur = next;
        }
        return prev;
    }
}