Algorithm
Problem Name: 206. Reverse Linked List
Given the head of a singly linked list, reverse the list, and return the reversed list.
Example 1:
Input: head = [1,2,3,4,5] Output: [5,4,3,2,1]
Example 2:
Input: head = [1,2] Output: [2,1]
Example 3:
Input: head = [] Output: []
Constraints:
- The number of nodes in the list is the range
[0, 5000]. -5000 <= Node.val <= 5000
Code Examples
#1 Code Example with C Programming
Code -
C Programming
struct ListNode* reverseList(struct ListNode* head) {
struct ListNode *a, *b, *c;
a = NULL;
b = head;
while (b) {
c = b->next;
b->next = a;
a = b;
b = c;
}
return a;
}
Input
head = [1,2,3,4,5]
Output
[5,4,3,2,1]
#2 Code Example with C++ Programming
Code -
C++ Programming
// Iterative
class Solution {
public:
ListNode* reverseList(ListNode* head) {
ListNode* pre(NULL), *cur(head), *next;
while(cur){
next = cur->next;
cur->next = pre;
pre = cur;
cur = next;
}
return pre;
}
};
// Recursive
class Solution {
public:
ListNode* reverseList(ListNode* head) {
if (!head || !head->next) return head;
ListNode* node = reverseList(head->next);
head->next->next = head;
head->next = NULL;
return node;
}
};
Input
head = [1,2,3,4,5]
Output
[5,4,3,2,1]
#3 Code Example with Java Programming
Code -
Java Programming
class Solution {
public ListNode reverseList(ListNode head) {
ListNode prev = null;
ListNode next = null;
ListNode curr = head;
while (curr != null) {
next = curr.next;
curr.next = prev;
prev = curr;
curr = next;
}
return prev;
}
}
Input
head = [1,2]
Output
[2,1]
#4 Code Example with Javascript Programming
Code -
Javascript Programming
const reverseList = function(head) {
if(head == null) return head
const pre = new ListNode(null, head)
let cur = head
while(cur.next) {
let tmp = pre.next
pre.next = cur.next
cur.next = cur.next.next
pre.next.next = tmp
}
return pre.next
};
Input
head = [1,2]
Output
[2,1]
#5 Code Example with Python Programming
Code -
Python Programming
class Solution:
def reverseList(self, head: ListNode, pre = None) -> ListNode:
if head:
nex = head.next
head.next = pre
return self.reverseList(nex, head) if nex else head
Input
head = []
Output
[]
#6 Code Example with C# Programming
Code -
C# Programming
namespace LeetCode
{
public class _0206_ReverseLinkedList
{
public ListNode ReverseList(ListNode head)
{
var dummyHead = new ListNode(-1);
var current = head;
while (current != null)
{
var next = current.next;
current.next = dummyHead.next;
dummyHead.next = current;
current = next;
}
return dummyHead.next;
}
}
}
Input
head = []
Output
[]