Merge two sorted list

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

Example 1:

Input: list1 = [1,2,4], list2 = [1,3,4]
Output: [1,1,2,3,4,4]

Example 2:

Input: list1 = [], list2 = []
Output: []

Example 3:

Input: list1 = [], list2 = [0]
Output: [0]

Constraints:

  • The number of nodes in both lists is in the range [0, 50]
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    .
  • -100 <= Node.val <= 100
  • Both list1 and list2 are sorted in non-decreasing order.

Solution 1: 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;
		}
}

Solution 2: Using iterative

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        else if(l2 == null) return l1;
        ListNode dummy = new ListNode(0);
        ListNode curr = dummy;
        while(l1 != null && l2!= null){
            if(l1.val <= l2.val){
                curr.next = l1;
                l1 = l1.next;
            }else {
                curr.next = l2;
                l2 = l2.next;
            }
            curr = curr.next;
        }
        curr.next = l1 == null? l2:l1;
        return dummy.next;
    }
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