Merge pull request #89 from version0chiro/Linked_List_Questions

added palindrome and DLL reverse for linked list

Author: smv1999

Data Structures/Linked Lists/check_if_linkedlist_is_a_palindrome.cpp

@@ -0,0 +1,97 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <iostream>
+using namespace std;
+
+
+/*
+Approch used: 
+-First traverse the LL to the middle node
+using floyd's algorithm. 
+-Than reverse the second half of the Linked list.
+-Reset the fast pointer to head and compare each node.
+
+-If Any does not match, the LL is not a palindrome.
+
+
+*/
+struct Node
+{
+    int data;
+    struct Node *next;
+    Node(int x)
+    {
+        data = x;
+        next = NULL;
+    }
+};
+
+class Solution
+{
+public:
+    //Function to check whether the list is palindrome.
+    Node *reverseList(Node *head)
+    {
+        Node *curr = head;
+        Node *temp = NULL;
+        Node *prev = NULL;
+        while (curr)
+        {
+            temp = curr->next;
+            curr->next = prev;
+            prev = curr;
+            curr = temp;
+        }
+
+        return prev;
+    }
+    bool isPalindrome(Node *head)
+    {
+        Node *slow = head;
+        Node *fast = head;
+        while (fast && fast->next)
+        {
+            fast = fast->next->next;
+            slow = slow->next;
+        }
+        slow = reverseList(slow);
+        fast = head;
+        while (slow)
+        {
+            if (slow->data != fast->data)
+            {
+                return false;
+            }
+            slow = slow->next;
+            fast = fast->next;
+        }
+
+        return true;
+    }
+};
+
+int main()
+{
+    int T, i, n, l, firstdata;
+    cin >> T;
+    while (T--)
+    {
+
+        struct Node *head = NULL, *tail = NULL;
+        cin >> n;
+        // taking first data of LL
+        cin >> firstdata;
+        head = new Node(firstdata);
+        tail = head;
+        // taking remaining data of LL
+        for (i = 1; i < n; i++)
+        {
+            cin >> l;
+            tail->next = new Node(l);
+            tail = tail->next;
+        }
+        Solution obj;
+        cout << obj.isPalindrome(head) << endl;
+    }
+    return 0;
+}

Data Structures/Linked Lists/reverse_a_double_linkedlist.cpp

@@ -0,0 +1,108 @@
+#include <bits/stdc++.h>
+
+/*
+Given a doubly linked list of n elements. The task is to reverse the doubly linked list.
+
+Example:
+Input: LinkedList: 3 <--> 4 <--> 5
+Output: 5 4 3
+*/
+
+using namespace std;
+
+struct Node
+{
+    int data;
+    Node * next;
+    Node * prev;
+    Node (int x)
+    {
+        data=x;
+        next=NULL;
+        prev=NULL;
+    }
+        
+};
+
+Node *newNode(int data)
+{
+    Node *temp=new Node(data);
+    
+    return temp;
+}
+
+
+void displayList(Node *head)
+{
+    while(head->next)
+    {
+        cout<<head->data<<" ";
+        head=head->next;
+    }
+    cout<<head->data;
+    
+    
+    
+}
+
+// main function for reversing the doubly linked list
+
+/*
+Approch:
+Simply just keep track of the previous node
+and the next node. 
+
+Almost similar to the singly linked list you have an extra pointer
+as prev to take care of 
+*/
+Node* reverseDLL(Node * head)
+{
+    Node* temp = NULL;
+    Node* curr = head;
+    while(curr){
+        temp = curr->prev;
+        curr->prev=curr->next;
+        curr->next = temp;
+        curr=curr->prev;
+    }
+    
+    if(temp){
+        head= temp->prev;
+    }
+    
+    return head;
+}
+
+
+
+int main() {
+	int t;
+	cin>>t;
+	while(t--)
+	{
+	    int n;
+	    cin>>n;
+	    Node *head=NULL, *tail=NULL;
+        int x;
+	    cin>>x;
+	    head = newNode(x);
+	    tail = head;
+	    
+	    for(int i=0;i<n - 1;i++)
+	    {
+	        cin>>x;
+	        Node* temp=newNode(x);
+	        tail->next=temp;
+	        temp->prev= tail;
+	        tail = temp;
+	    }
+	    head=reverseDLL(head);
+	    
+	    
+	    displayList(head);
+	    
+	    cout<<endl;
+	}
+	return 0;
+}
+