notes/CS notes/pages/Leetcode Reverse-Linked-List.md

# Leetcode Reverse-Linked-List

#### 2022-06-15 22:07

---
##### Algorithms:
#algorithm #recursion #iteration 
##### Data structures:
#DS #linked_list 
##### Difficulty:
#leetcode  #coding_problem #difficulty-medium
##### Lists:
#CS_list_need_understanding #CS_list_need_practicing 
##### Related topics:
```expander
tag:#linked_list
```

 

##### Links:
- [Link to problem](https://leetcode.com/problems/reverse-linked-list/)
___
### Problem
Given the `head` of a singly linked list, reverse the list, and return _the reversed list_.
#### Examples
**Example 1:**

![](https://assets.leetcode.com/uploads/2021/02/19/rev1ex1.jpg)

```markdown
**Input:** head = [1,2,3,4,5]
**Output:** [5,4,3,2,1]
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2021/02/19/rev1ex2.jpg)

```markdown
**Input:** head = [1,2]
**Output:** [2,1]
```

**Example 3:**

```markdown
**Input:** head = []
**Output:** []
```

#### Constraints
-   The number of nodes in the list is the range `[0, 5000]`.
-   `-5000 <= Node.val <= 5000`

### Thoughts

I thought a slow O(n ^ 2) hybrid solution, while there are better algorithms, using in-place insert, or recursion.

The in place insert is easier to understand, and simple to implement, using a very clever trick.

### Solution
I've referred to this guy: https://leetcode.com/problems/reverse-linked-list/discuss/58130/C%2B%2B-Iterative-and-Recursive

This code is hard to understand.
==TODO==: Make my own version
```cpp
class Solution {
public:
  ListNode *reverseList(ListNode *head) {
    ListNode *pre = new ListNode(0), *cur = head;
    // pre is before head, and insert any element after pre.
    pre->next = head;
    while (cur && cur->next) {
      // temp points to head
      ListNode *temp = pre->next;
      // Move cur->next after pre.
      pre->next = cur->next;
      // Fix pointers, because cur->next is unchanged when changing position.
      cur->next = cur->next->next;
      pre->next->next = temp;
    }
    return pre->next;
  }
};
```

Recursion:
```cpp
class Solution {
public:
  ListNode *reverseList(ListNode *head) {
    // Base case: reached the end of list
    if (!head || !(head->next)) {
      return head;
    }

    // node is the end of linked list, which stays the same and untouched
    ListNode *node = reverseList(head->next);
    // change head->next
    head->next->next = head;
    head->next = NULL;
    return node;
  }
};
```