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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        
        ListNode result = new ListNode(0);
        ListNode curr = result;
        
        // add smallest from both lists
        while (l1 != null) {    
            
            if (l2 == null || l1.val <= l2.val) {
                curr.next = l1;
                curr = curr.next;
                l1 = l1.next;
            } else {
                curr.next = l2;
                curr = curr.next;
                l2 = l2.next;
            }
        }
        
        // finish rest of l2
        while (l2 != null) {
            curr.next = l2;
            curr = curr.next;
            l2 = l2.next;
        }
        
        return result.next;
    }
}