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Leetcode 21 Merge Two Sorted Lists.

Problem 21 Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Not a hard one. But I spend a lot of time on this, cause I misunderstand the question, I used to think it should be like l1->l2->l1, can't use a new list, but that is not right.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {

            if(l1==null){
                return l2;
            }
            if(l2==null){
                return l1;
            }
            ListNode l3=new ListNode(-1);
            if(l1.val < l2.val){
                l3=l1;
                l1=l1.next;
            }else{
                l3=l2;
                l2=l2.next;
            }
            ListNode head=l3;


            while(l1!=null&&l2!=null){
                if(l1.val < l2.val){
                l3.next=l1;
                l3=l3.next;
                l1=l1.next;
                }else{
                l3.next=l2;
                l3=l3.next;
                l2=l2.next;
            }
        }

        if(l1==null){
            l3.next=l2;
        }

        if(l2==null){
            l3.next=l1;
        }
        return head;
    }
}
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