# 21 Merge Two Sorted Lists

## 21. [Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/description/)

## 1. Question

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.

**Example:**

```
Input:
1->2->4, 1->3->4

Output:
1->1->2->3->4->4
```

## 2. Implementation

```java
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode p1 = l1, p2 = l2, curNode = dummy;

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

        if (p1 != null) {
            curNode.next = p1;
        }
        else {
            curNode.next = p2;
        }
        return dummy.next;
    }
}
```

## 3. Time & Space Complexity

时间复杂度: O(n), 空间复杂度O(1)
