# Leetcode No.21
**[題目連結](https://leetcode.com/problems/merge-two-sorted-lists/description/)**
## JavaScript
```javascript=
/**
* Definition for singly-linked list.
* function ListNode(val, next) {
* this.val = (val===undefined ? 0 : val)
* this.next = (next===undefined ? null : next)
* }
*/
/**
* @param {ListNode} list1
* @param {ListNode} list2
* @return {ListNode}
*/
var mergeTwoLists = function(list1, list2) {
if (!list1){
return list2;
}
if (!list2){
return list1;
}
var current_node =new ListNode(0,null);
var result = current_node;
while (list1 != null && list2 != null){
if (list1.val <= list2.val){
current_node.next = new ListNode(list1.val,null);
current_node = current_node.next;
list1 = list1.next;
}
else{
current_node.next = new ListNode(list2.val,null);
current_node = current_node.next;
list2 = list2.next;
}
}
if (list1 == null){
current_node.next = list2;
}
else{
current_node.next = list1;
}
result = result.next;
return result;
}
```