LeetCode - Partition List

Partition List

//
public ListNode partition(ListNode head, int x) {
        if(head == null) return head;
        ListNode dummyNode = new ListNode(0);
        dummyNode.next = head;
        ListNode first = dummyNode, second = null;
        while(head != null){
            if(head.val >= x){
                if(second == null){
                    second = head;
                } else {
                    second = second.next;
                }
                head = head.next;
            } else{
                if(second == null){
                    head = head.next;
                } else {
                    ListNode next = head.next;
                    head.next = first.next;
                    first.next = head;
                    head = next;
                    second.next = head;
                }
                first = first.next;
            }
        }
        return dummyNode.next;
    }
    
//
public ListNode partition(ListNode head, int x) {
        if(head==null) return null;
        ListNode helper = new ListNode(0);
        helper.next = head;
        ListNode pre = helper;
        ListNode cur = helper;
        while(cur.next!=null){
            if(cur.next.val<x){
                if(pre!=cur){
                    ListNode next = cur.next.next;
                    cur.next.next = pre.next;
                    pre.next = cur.next;
                    cur.next = next;
                    
                }else{
                    cur = cur.next;
                }
                pre = pre.next;
            }else{
                cur = cur.next;
            }
        }
        return helper.next;
    }
    
//
public ListNode partition(ListNode head, int x) {
        if(head==null) return null;
        ListNode helper1 = new ListNode(0);
        ListNode helper2 = new ListNode(0);
        ListNode pre1 = helper1;
        ListNode pre2 = helper2;
        while(head!=null){
            if(head.val<x){
                helper1.next = head;
                helper1 = helper1.next;
            }else{
                helper2.next = head;
                helper2 = helper2.next;
            }
            head = head.next;
        }
        helper2.next = null;
        helper1.next = pre2.next;
        return pre1.next;
    }