leetcode -- Merge Two Sorted Lists

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.

Solution 1:

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 p1 = l1;
        ListNode p2 = l2;
        ListNode answer = null;

        if (p1.val<p2.val){
            answer = p1;
            p1 = p1.next;
        }else{
            answer = p2;
            p2 = p2.next;
        }
        ListNode p = answer;

        while(p1!=null & p2!=null){
            if(p1.val < p2.val){
                p.next = p1;
                p1 = p1.next;
                p = p.next;
            }else{
                p.next = p2;
                p2 = p2.next;
                p = p.next;
            }
        }

        if (p1!=null){
            p.next = p1;
        }

        if(p2!=null){
            p.next = p2;
        }
        return answer;
    }
}

Solution 2:

Using recursion

public ListNode mergeTwoLists(ListNode l1, ListNode l2){
        if(l1==null)
            return l2;
        if(l2==null)
            return l1;
        if(l1.val < l2.val){
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        }else{
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    }