# Problem

Given a linked list, determine if it has a cycle in it.

Can you solve it without using extra space?

# Solution1: HashTable

Time complexity: O(n)

Space complexity: O(n)

# Solution2: Fast + Slow pointers

Time complexity: O(n)

Space complexity: O(1)

# Problem

Design your implementation of the linked list. You can choose to use the singly linked list or the doubly linked list. A node in a singly linked list should have two attributes: val and nextval is the value of the current node, and next is a pointer/reference to the next node. If you want to use the doubly linked list, you will need one more attribute prev to indicate the previous node in the linked list. Assume all nodes in the linked list are 0-indexed.

• get(index) : Get the value of the index-th node in the linked list. If the index is invalid, return -1.
• addAtHead(val) : Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list.
• addAtTail(val) : Append a node of value val to the last element of the linked list.
• addAtIndex(index, val) : Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted.
• deleteAtIndex(index) : Delete the index-th node in the linked list, if the index is valid.

Example:

Note:

• All values will be in the range of [1, 1000].
• The number of operations will be in the range of [1, 1000].

Keep tracking head and tail of the list.

Time Complexity:

deleteAtIndex O(index)

Space complexity: O(1)

C++

Tracking head/tail and size of the list.

v2

Python3

Java

# Problem

Given a non-empty, singly linked list with head node head, return a middle node of linked list.

If there are two middle nodes, return the second middle node.

Example 1:

Input: [1,2,3,4,5]
Output: Node 3 from this list (Serialization: [3,4,5])
The returned node has value 3.  (The judge's serialization of this node is [3,4,5]).
Note that we returned a ListNode object ans, such that:
ans.val = 3, ans.next.val = 4, ans.next.next.val = 5, and ans.next.next.next = NULL.


Example 2:

Input: [1,2,3,4,5,6]
Output: Node 4 from this list (Serialization: [4,5,6])
Since the list has two middle nodes with values 3 and 4, we return the second one.


Note:

• The number of nodes in the given list will be between 1 and 100.

# Solution: Slow + Fast Pointers

Time complexity: O(n)

Space complexity: O(1)

# Problem

Sort a linked list in O(n log n) time using constant space complexity.

Example 1:

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


Example 2:

Input: -1->5->3->4->0
Output: -1->0->3->4->5

# Solution: Merge Sort

Top-down (recursion)

Time complexity: O(nlogn)

Space complexity: O(logn)

## Python3

bottom up

Time complexity: O(nlogn)

Space complexity: O(1)

# Problem

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

# Solution: Simulation

Simulate the addition, draw two numbers (one each) from l1, l2, if list is empty draw 0.

Using a dummy head makes things easier.

Time complexity: O(max(l1,l2))

Space complexity: O(max(l1,l2))

# Related Problems

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