ListNode.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {

         ListNode dummyNode = new ListNode(0);
    ListNode tail = dummyNode;
    while(true) 
    {
        if(list1 == null)
        {
            tail.next = list2;
            break;
        }
        if(list2 == null)
        {
            tail.next = list1;
            break;
        }
          
        if(list1.val <= list2.val)
        {
            tail.next = list1;
            list1 = list1.next;
        } 
        else
        {
            tail.next = list2;
            list2 = list2.next;
        }
        tail = tail.next;
    }
    return dummyNode.next;
        
    }
}