Partition List

Given a linked list and a value x, partition it such that all nodes less than x come before nodes greater than or equal to x.

You should preserve the original relative order of the nodes in each of the two partitions.

Example:

Input: head = 1->4->3->2->5->2, x = 3
Output: 1->2->2->4->3->5

class Solution {
    public ListNode partition(ListNode head, int x) {
        ListNode smallerHead = new ListNode(0), biggerHead = new ListNode(0);
        ListNode smaller = smallerHead, bigger = biggerHead;
        while (head != null) {
            if (head.val < x)
                smaller = smaller.next = head;
            else
                bigger = bigger.next = head;
            head = head.next;
        }
        // no need for extra check because of fake heads
        smaller.next = biggerHead.next; // join bigger after smaller
        bigger.next = null; // cut off anything after bigger
        return smallerHead.next;
    }
}