# 21. Merge Two Sorted Lists

time: O(n)

space: O(1), O(n)

#### iteration

```java
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode prevHead = new ListNode(-1);
        ListNode prev = prevHead;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                prev.next = l1;
                l1 = l1.next;
            } else {
                prev.next = l2;
                l2 = l2.next;
            }
            prev = prev.next;
        }
        prev.next = (l1 == null) ? l2 : l1;
        return prevHead.next;
    }
}
```

#### recursion

```java
class Solution {
    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;
        }
        l2.next = mergeTwoLists(l1, l2.next);
        return l2;
    }
}
```

## JS

```javascript
var mergeTwoLists = function(l1, l2) {
    let head = new ListNode();
    let cur = head;
    while (l1 != null && l2 != null) {
        if (l1.val < l2.val) {
            cur.next = l1;
            l1 = l1.next;
        } else {
            cur.next = l2;
            l2 = l2.next;
        }
        cur = cur.next;
    }
    cur.next = l1 ? l1 : l2;

    return head.next;
};
```
