21. Merge Two Sorted Lists.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) {
        
        if(list1== null) return list2;
        if(list2 == null) return list1;
        
        ListNode dummy = new ListNode(-1);
        ListNode head = dummy, tail = dummy;
        
        while(list1!=null && list2!=null)
        {
            if(list1.val <= list2.val)
            {
                tail.next = list1;
                list1 = list1.next;
            }
            else
            {
                tail.next = list2;
                list2 = list2.next;
            }
            
            tail = tail.next;
        }
        if(list1!=null) tail.next = list1;
        else tail.next = list2;
        
        return head.next;
        
    }
}