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# 0021. Merge Two Sorted Lists

<https://leetcode.com/problems/merge-two-sorted-lists>

## Description

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

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/10/03/merge_ex1.jpg)

```
**Input:** l1 = [1,2,4], l2 = [1,3,4]
**Output:** [1,1,2,3,4,4]
```

**Example 2:**

```
**Input:** l1 = [], l2 = []
**Output:** []
```

**Example 3:**

```
**Input:** l1 = [], l2 = [0]
**Output:** [0]
```

**Constraints:**

* The number of nodes in both lists is in the range `[0, 50]`.
* `-100 <= Node.val <= 100`
* Both `l1` and `l2` are sorted in **non-decreasing** order.

## ac

```java
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;
    }
```