how to remove duplicate elements from the linked list - c++

Question

I have to make a program to count the number of occurrences of a given key in a singly linked
list and then delete all the occurrences. For example, if given linked list is 1->2->1->2->1->3->1 and given key is 1, then output should be 4. After deletion of all the
occurrences of 1, the linked list is 2->2->3.
I tried making a program that removes duplicate elements from a linked list and count the number of occurrence of the duplicate element
output for my program is
Your linked list is 1 2 1 2 1 3 1
Enter a number: 1
Number of occurance is: 4
After deleting repeated elements:
Your linked list is 2 2 1 3 1
I am going to add third case to remove element from the end soon but I need help in this function . Here I am only able to remove one '1' why is this happening please help
struct Node* ptr2= head;
struct Node* ptr3= head->next;
while(ptr3!=NULL)
{
if(ptr3->data==x)
{
ptr2->next=ptr3->next;
}
ptr2=ptr2->next;
ptr3=ptr3->next;
}
cout<<"Number of occurance is: "<<count<<endl;
return head;
whole program is :-
#include<iostream>
using namespace std;
struct Node
{
int data;
struct Node* next;
};
void traversal(struct Node* head)
{
cout<<"Your linked list is ";
struct Node* ptr = head;
while(ptr!=NULL)
{
cout<<ptr->data<<" ";
ptr=ptr->next;
}
}
struct Node* deleterepeated(struct Node* head, int x)
{
int s=0;//number of elements in linked list
struct Node* p = head;
while(p!=NULL)
{
p=p->next;
s++;
}
struct Node* ptr = head;
int count=0;
while(ptr!=NULL)
{
if(ptr->data == x)
{
if(count==0)
{
head=ptr->next;
}
count++;
}
ptr=ptr->next;
}
struct Node* ptr2= head;
struct Node* ptr3= head->next;
while(ptr3!=NULL)
{
if(ptr3->data==x)
{
ptr2->next=ptr3->next;
}
ptr2=ptr2->next;
ptr3=ptr3->next;
}
cout<<"Number of occurance is: "<<count<<endl;
return head;
}
int main()
{
struct Node* head;
struct Node* val1;
struct Node* val2;
struct Node* val3;
struct Node* val4;
struct Node* val5;
struct Node* val6;
head= (struct Node*)malloc(sizeof(struct Node));
val1= (struct Node*)malloc(sizeof(struct Node));
val2= (struct Node*)malloc(sizeof(struct Node));
val3= (struct Node*)malloc(sizeof(struct Node));
val4= (struct Node*)malloc(sizeof(struct Node));
val5= (struct Node*)malloc(sizeof(struct Node));
val6= (struct Node*)malloc(sizeof(struct Node));
head->data=1;
val1->data=2;
val2->data=1;
val3->data=2;
val4->data=1;
val5->data=3;
val6->data=1;
head->next=val1;
val1->next=val2;
val2->next=val3;
val3->next=val4;
val4->next=val5;
val5->next=val6;
val6->next=NULL;
traversal(head);
cout<<endl;
cout<<"Enter a number: ";
int x;
cin>>x;
head=deleterepeated(head,x);
cout<<"After deleting repeated elements: "<<endl;
traversal(head);
return 0;
}

Answer 1

It is very easy to do if you use the standard linked list implementation:
#include <iostream>
#include <list>
int main() {
std::list<int> list{1, 2, 1, 2, 1, 3, 1};
int nOnes = 0;
for (auto it = list.begin(), end = list.end(); it != end; /*no op*/) {
if (*it == 1) {
++nOnes;
it = list.erase(it); // remove element idiom
} else {
++it;
}
}
std::cout << "Removed " << nOnes << " item(s)\n"
<< "Remains: \n";
for (auto const e : list) {
std::cout << e << '\n';
}
}
As a side-note, it is even nicer with a vector:
#include <algorithm>
#include <vector>
int main() {
std::vector<int> vec{1, 2, 1, 2, 1, 3, 1};
auto const nOnes = vec.end() - std::remove(vec.begin(), vec.end(), 1);
vec.resize(vec.size() - nOnes);
}

Answer 2

so , you have a little tiny mistakes in your code , I edited it out , all of the edits are in the function named deleterepeated and here is my solution , not the best but it will work for your case :
EDIT: I removed unnecessary things in the code to be only one big loop , also I found a bug in my previous code and edit it out here in the new version , this is only the deleterepeated function
struct Node* deleterepeated(struct Node* head, int x)
{
int count = 0;
struct Node* ptr2 = head;
struct Node* ptr3 = ptr2->next;
struct Node* temp;
while (ptr3 != NULL)
{
if (head && head->data == x) // here is the part of deleting the head if it matches
{
temp = head;
head = head->next;
free(temp);
ptr2 = head;
ptr3 = head->next;
}
else if (ptr3 && ptr3->data == x)
{
temp = ptr3;
ptr2->next = ptr3->next;
free(temp); // you have to free memory to avoid memory leak
ptr3 = ptr2->next;
count++;
}
else
{
ptr2 = ptr2->next;
ptr3 = ptr3->next;
}
}
cout << "Number of occurance is: " << count << endl;
return head;
}
picture of the results: