Everything About Linked List With Python Code and HackerRank/LeetCode Solutions

Updated August 2026 — full tutorial restored for this URL.

This is a practical linked list guide in Python: structure, core operations, and patterns that show up on HackerRank / LeetCode. Also see How to Reverse a Linked List.

1. Singly linked list basics
2. Insert and delete
3. Traverse and reverse
4. Two-pointer patterns (middle, cycle)
5. Merge two sorted lists
6. Practice checklist

1. Singly linked list basics

class Node:
    def __init__(self, val, next=None):
        self.val = val
        self.next = next

class LinkedList:
    def __init__(self):
        self.head = None

Arrays give O(1) index access; linked lists give O(1) insert/delete at a known node (after you find it).

2. Insert and delete

def push_front(self, val):
    self.head = Node(val, self.head)

def delete_val(self, val):
    dummy = Node(0, self.head)
    cur = dummy
    while cur.next:
        if cur.next.val == val:
            cur.next = cur.next.next
            break
        cur = cur.next
    self.head = dummy.next

Dummy nodes simplify edge cases at the head.

3. Traverse and reverse

def to_list(head):
    out = []
    while head:
        out.append(head.val)
        head = head.next
    return out

def reverse(head):
    prev = None
    while head:
        nxt = head.next
        head.next = prev
        prev, head = head, nxt
    return prev

4. Two-pointer patterns (middle, cycle)

def middle(head):
    slow = fast = head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
    return slow

def has_cycle(head):
    slow = fast = head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
        if slow is fast:
            return True
    return False

Floyd’s cycle detection is a classic interview question (LeetCode 141).

5. Merge two sorted lists

def merge(l1, l2):
    dummy = Node(0)
    cur = dummy
    while l1 and l2:
        if l1.val <= l2.val:
            cur.next, l1 = l1, l1.next
        else:
            cur.next, l2 = l2, l2.next
        cur = cur.next
    cur.next = l1 or l2
    return dummy.next

6. Practice checklist

• Reverse list / reverse in k-groups
• Remove nth from end (two pointers)
• Detect/start of cycle
• Merge k sorted lists (heap)
• Copy list with random pointer

Draw pointers on paper before coding — most bugs are lost references, not syntax.