1.intro
A linked list is a linear data structure, in which the elements are not stored at contiguous memory locations. The elements in a linked list are linked using pointers as shown in the below image:
In simple words, a linked list consists of nodes where each node contains a data field and a reference(link) to the next node in the list.
```java
```
All programs discussed in this post consider following representations of linked list .
// Linked List Class class LinkedList
```java
{
Node head; // head of list /* Node Class */class Node
{
int data;
Node next;
// Constructor to create a new node Node(intd) {data = d; next =null; }
}
}
```
Add a node at the front: (A 4 steps process)
The new node is always added before the head of the given Linked List. And newly added node becomes the new head of the Linked List. For example if the given Linked List is 10->15->20->25 and we add an item 5 at the front, then the Linked List becomes 5->10->15->20->25. Let us call the function that adds at the front of the list is push(). The push() must receive a pointer to the head pointer, because push must change the head pointer to point to the new node (Seethis)
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/* This function is in LinkedList class. Inserts a
new Node at front of the list. This method is
defined inside LinkedList class shown above */publicvoidpush(int new_data)
{
/* 1 & 2: Allocate the Node &
Put in the data*/ Node new_node =new Node(new_data);
/* 3. Make next of new Node as head */ new_node.next = head;
/* 4. Move the head to point to new Node */ head = new_node;
}
Add a node after a given node: (5 steps process)
We are given pointer to a node, and the new node is inserted after the given node.
/* This function is in LinkedList class.
Inserts a new node after the given prev_node. This method is
defined inside LinkedList class shown above */publicvoidinsertAfter(Node prev_node,int new_data)
{
/* 1. Check if the given Node is null */if(prev_node ==null)
{
System.out.println("The given previous node cannot be null");
return;
}
/* 2. Allocate the Node &
3. Put in the data*/ Node new_node =new Node(new_data);
/* 4. Make next of new Node as next of prev_node */ new_node.next = prev_node.next;
/* 5. make next of prev_node as new_node */ prev_node.next = new_node;
}
Add a node at the end: (6 steps process)
The new node is always added after the last node of the given Linked List. For example if the given Linked List is 5->10->15->20->25 and we add an item 30 at the end, then the Linked List becomes 5->10->15->20->25->30.
Since a Linked List is typically represented by the head of it, we have to traverse the list till end and then change the next of last node to new node.
/* Appends a new node at the end. This method is
defined inside LinkedList class shown above */publicvoidappend(int new_data)
{
/* 1. Allocate the Node &
2. Put in the data
3. Set next as null */ Node new_node =new Node(new_data);
/* 4. If the Linked List is empty, then make the
new node as head */if(head ==null)
{
head =new Node(new_data);
return;
}
/* 4. This new node is going to be the last node, so
make next of it as null */ new_node.next =null;
/* 5. Else traverse till the last node */ Node last = head;
while(last.next !=null)
last = last.next;
/* 6. Change the next of last node */ last.next = new_node;
return;
}
Deleting a node
Let us formulate the problem statement to understand the deletion process.Given a ‘key’, delete the first occurrence of this key in linked list.
To delete a node from linked list, we need to do following steps.
1) Find previous node of the node to be deleted.
2) Change the next of previous node.
3) Free memory for the node to be deleted.
/* Given a key, deletes the first occurrence of key in linked list */voiddeleteNode(int key)
{
// Store head node Node temp = head, prev =null;
// If head node itself holds the key to be deleted if(temp !=null&& temp.data == key)
{
head = temp.next;// Changed head return;
}
// Search for the key to be deleted, keep track of the
// previous node as we need to change temp.next while(temp !=null&& temp.data != key)
{
prev = temp;
temp = temp.next;
}
// If key was not present in linked list if(temp ==null)return;
// Unlink the node from linked list prev.next = temp.next;
}
Detect loop in a linked list
Given a linked list, check if the linked list has loop or not. Below diagram shows a linked list with a loop.
Use Hashing:
Traverse the list one by one and keep putting the node addresses in a Hash Table. At any point, if NULL is reached then return false and if next of current node points to any of the previously stored nodes in Hash then return true.
// Java program to detect loop in a linked list importjava.util.*; publicclass LinkedList {
staticNode head;// head of list /* Linked list Node*/staticclass Node {
int data;
Node next;
Node(int d)
{
data = d;
next =null;
}
}
/* Inserts a new Node at front of the list. */staticpublicvoidpush(int new_data)
{
/* 1 & 2: Allocate the Node &
Put in the data*/ Node new_node =new Node(new_data);
/* 3. Make next of new Node as head */ new_node.next = head;
/* 4. Move the head to point to new Node */ head = new_node;
}
// Returns true if there is a loop in linked
// list else returns false. staticboolean detectLoop(Node h)
{
HashSet s =newHashSet();
while(h !=null) {
// If we have already has this node
// in hashmap it means their is a cycle
// (Because you we encountering the
// node second time). if (s.contains(h))
returntrue;
// If we are seeing the node for
// the first time, insert it in hash s.add(h);
h = h.next;
}
returnfalse;
}
/* Driver program to test above function */publicstaticvoid main(String[] args)
{
LinkedList llist =new LinkedList();
llist.push(20);
llist.push(4);
llist.push(15);
llist.push(10);
/*Create loop for testing */ llist.head.next.next.next.next = llist.head;
if (detectLoop(head))
System.out.println("Loop found");
else System.out.println("No Loop");
}
}
Floyd’s Cycle-Finding Algorithm:This is the fastest method and has been described below:
Traverse linked list using two pointers.
Move one pointer(slow_p) by one and another pointer(fast_p) by two.
If these pointers meet at the same node then there is a loop. If pointers do not meet then linked list doesn’t have a loop
.
Below image shows how the detectloop function works in the code :
Implementation of Floyd’s Cycle-Finding Algorithm:
// Java program to detect loop in a linked list class LinkedList {
Node head; // head of list /* Linked list Node*/class Node {
int data;
Node next;
Node(int d)
{
data = d;
next =null;
}
}
/* Inserts a new Node at front of the list. */publicvoidpush(int new_data)
{
/* 1 & 2: Allocate the Node &
Put in the data*/ Node new_node =new Node(new_data);
/* 3. Make next of new Node as head */ new_node.next = head;
/* 4. Move the head to point to new Node */ head = new_node;
}
int detectLoop()
{
Node slow_p = head, fast_p = head;
while(slow_p !=null&& fast_p !=null&& fast_p.next !=null) {
slow_p = slow_p.next;
fast_p = fast_p.next.next;
if(slow_p == fast_p) {
System.out.println("Found loop");
return1;
}
}
return0;
}
/* Drier program to test above functions */publicstaticvoid main(String args[])
{
LinkedList llist =new LinkedList();
llist.push(20);
llist.push(4);
llist.push(15);
llist.push(10);
/*Create loop for testing */ llist.head.next.next.next.next = llist.head;
llist.detectLoop();
}
}