Tech Interview prep practice - answers

partitionList.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode partition(ListNode head, int x) {
        
        if (head == null || head.next == null) {
            return head;
        }
        
        ArrayList<Integer> smallerList = new ArrayList<Integer>();
        ArrayList<Integer> largerList = new ArrayList<Integer>();
        ListNode curr = head;
        while (head != null) {
            if (head.val < x) {
                smallerList.add(head.val);
            } else {
                largerList.add(head.val);
            }
            head = head.next;
        }
        
        ListNode newHead = curr;
        for (int i : smallerList) {
            curr.val = i;
            curr = curr.next;    
        }
        for (int i:largerList) {
            curr.val = i;
            curr= curr.next;
        }
        return newHead;
    }
}