Merge Two Sorted Lists

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(l1 == null && l2 == null)
            return null;
        if(l1 == null)
            return l2;
        if(l2 == null)
            return l1;
        
        ListNode header = null;
        ListNode runner = null;
        while(l1 != null && l2 != null)
        {
            if(l1.val < l2.val)
            {
                if(header == null){
                    header = l1;
                    runner = header;
                }
                else{
                    runner.next = l1;
                    runner = runner.next;
                }
                l1 = l1.next;
            }
            else
            {
                if(header == null){
                    header = l2;
                    runner = header;
                }
                else{
                    runner.next = l2;
                    runner = runner.next;
                }
                l2 = l2.next;
            }
        }
        if(l1 != null)
            runner.next = l1;
        if(l2 != null)
            runner.next = l2;
        return header;
        
    }
}