1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
    //new a head to return;
    ListNode head = null;
    //check two heads are available to merge
    if(list1 == null){
        return list2;
    }
    if(list2 == null){
        return list1;
    }
    if(list1 == null && list2 == null){
        return head;
    }

    //build three pointers, cur for the last element in the sorted linkedlist;
    //cur1 for the first element in the list1;
    //cur2 for the first element in the list2;
    //we can decide the head now;
    ListNode cur1;
    ListNode cur2;
    if(list1.val > list2.val){
        head = list2;
        cur1 = list1;
        cur2 = list2.next;
        
    }
    else{
        head = list1;
        cur1 = list1.next;
        cur2 = list2;
    }
    ListNode cur = head;

    //while loop to check which one is biger, until one of pointers of one list is null;
    while(cur1 != null && cur2 != null){
        if(cur1.val < cur2.val){
            cur.next = cur1;
            cur = cur.next;
            cur1 = cur1.next;
        }
        else{
            cur.next = cur2;
            cur = cur.next;
            cur2 = cur2.next;
        }
    }
    //check which list pointer is null; direct use 'cur' point to the pointer which still has linkedlist left;
    if(cur1 != null){
        cur.next = cur1;
    }
    else{
        cur.next = cur2;
    }
    return head;
}