merge sort easy and best program and algorithm in c++

WHY VECTOR?

The better approach over an array is the vector. This may sound bizarre to the newbies, learning c++. 
Let me ask you a question before we walk through the code. 
Let us take

                          int *arr=new int[n];
this creates n integer space holders in heap, pretty intuitive right.
Assume that we have filled up the integer space holders with some integer values and we pass this array arr to a function. 
Let the function be 
auto fun(int a[])
//what is a here, well your quick answer is it's an integer array  X---you are wrong buddy.
It is an integer pointer.It's  size depends on the architecture.
auto fun(int a[])
{int n=sizeof(a)/sizeof(a[0]);/*this gives you different answer in different computers based on the its architecture, whether it's a 16 or 32 or 64 bit*/
}
Because we are actually calculating the size of integer pointer.
sizeof(int *)/sizeof(int);
To understand this better see this post
 To avoid all these in-deliberate mistakes, I have chosen the vector collection.COOL right...

Merge Sort source code using vector collection in c++

#include<iostream>

#include<vector>

using namespace std;

class MergeSort

{

public:

void Merge(vector<int>&arr) //pass by reference to reflect the values in the array

                  {

int n=arr.size();

if(n<2)

return;  //base case

                     //recursion part

int mid=n/2;

vector<int>l;

vector<int>r;

for(int i=0;i<mid;i++)

l.push_back(arr[i]);

for(int i=0;i<(n-mid);i++)

r.push_back(arr[mid+i]);

Merge(l);

Merge(r);

MergeUtil(arr,l,r);

l.clear();//To clear the elements of the vector

r.clear();

}

void MergeUtil(vector<int>&a,vector<int>&l,vector<int>&r) //pass by reference

              {

int i,j,k;

i=j=k=0;

while(i<l.size()&&j<r.size() ){

if(l[i]<=r[j])

a[k++]=l[i++];

else

a[k++]=r[j++];

}

while(i<l.size() )

a[k++]=l[i++];

while(j<r.size() )

a[k++]=r[j++];

}

};

int main(){int n;cin>>n;

vector<int>arr;int d;

    for(int i=0;i<n;i++){

    cin>>d;

arr.push_back(d);

}

MergeSort m;

m.Merge(arr);

for(int i=0;i<arr.size();i++)

cout<<arr[i];

}

Please do see \this documentation on when to use vector