merge2sortlist.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */

/* Output is still a sorted list. */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1==null && l2==null) return null;
        if(l1==null) return l2;
        if(l2==null) return l1;
        
        ListNode head = new ListNode(0);
        ListNode temp = head;
        
        while(l1!=null && l2!=null){
            if(l1.val>l2.val){
                temp.next=l2;
                l2=l2.next;
            }else{
                temp.next=l1;
                l1=l1.next;
            }
            temp=temp.next;
        }
        
        if(l1 != null){
            temp.next = l1;
        }
        if(l2 != null){
            temp.next = l2;
        }
        
        return head.next;
    }
}