# 21.Merge-Two-Sorted-Lists

## 21. Merge Two Sorted Lists

## 题目地址

<https://www.lintcode.com/problem/merge-two-sorted-lists>

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

## 题目描述

```
Merge two sorted (ascending) linked lists and return it as a new sorted list. The new sorted list should be made by splicing together the nodes of the two lists and sorted in ascending order.
```

## 代码

### Approach 1: Iteration

**Intuition**

We can achieve the same idea via iteration by assuming that `l1` is entirely less than `l2` and processing the elements one-by-one, inserting elements of `l2` in the necessary places in `l1`.

**Complexity Analysis**

* Time complexity : O(n + m)
* Space complexity : O(1)

```java
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    ListNode dummy = new ListNode(0);
    ListNode curr = dummy;

    while (l1 != null && l2 != null) {
      if (l1.val > l2.val){
        curr.next = l2;
        l2 = l2.next;
      } else {
        curr.next = l1;
        l1 = l1.next;
      }

      curr = curr.next;
    }

    curr.next = (l1 != null) ? l1 : l2;

    return dummy.next;
  }
}
```

### Approach 2: Recursion

**Intuition**

We can recursively define the result of a `merge` operation on two lists as the following (avoiding the corner case logic surrounding empty lists):

list1.next = merge(list1.next, list2) list1.val < list2.val

list2.next = merge(list1, list2.next) otherwise

Namely, the smaller of the two lists' heads plus the result of a `merge` on the rest of the elements.

**Complexity Analysis**

* Time complexity : O(n + m)
* Space complexity : O(n + m)

  The first call to `mergeTwoLists` does not return until the ends of both `l1` and `l2` have been reached, so n + m stack frames consume O(n + m)space.

```java
class Solution {
  public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    if (l1 == null) {
      return l2;
    } else if (l2 == null) {
      return l1;
    } else if (l1.val < l2.val) {
      l1.next = mergeTwoLists(l1.next, l2);
      return l1;
    } else {
      l2.next = mergeTwoLists(l1, l2.next);
      return l2;
    }
  }
}
```


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