Confusing error when deleting dynamically allocated array in C++ - c++

Question

I am trying to implement a simple merge sort algorithm. What I am very confusing is that
I keep getting the following error message right after the "array2" is deleted.
"
free(): invalid next size (fast)
"
Please advise. Thank you very much!
#include <iostream>
#include <limits.h>
using namespace std;
void merge_sort(int*,int,int);
int main(){
//cout << "Max int: " << INT_MAX <<endl;
int n;
cin >> n;
int* array = new int(n+1);
for (int i=1; i<=n; i++)
cin >> array[i];
merge_sort(array,1,n);
cout << "--------------------------------------------" <<endl;
for (int i=1; i<=n; i++)
cout << array[i] <<endl;
}
void merge_sort(int* array,int p,int r){
cout << p << ' ' << r <<endl;
if (p == r)
return;
int q = int((p+r)/2);
merge_sort(array,p,q);
merge_sort(array,q+1,r);
//(p..q) and (q+1 .. r) sorted, then merge this two sorted array
int n1 = q-p+1;
int n2 = r-q;
cout << "Mark1 " <<n1<<' '<<n2<<endl;
int *array1;
array1 = new int(n1+1);
int *array2;
array2 = new int(n2+1);
for (int i=p; i<=q; i++)
array1[i-p] = array[i];
for (int i=q+1; i<=r; i++)
array2[i-q-1] = array[i];
array1[n1] = INT_MAX;
array2[n2] = INT_MAX; //CONSTANT, serve as sentinel
int p1 = 0;
int p2 = 0;
cout << "Mark2" << endl;
for (int i=p; i<=r; i++){
if (array1[p1]<array2[p2]){
array[i] = array1[p1];
p1++;
}else{
array[i] = array2[p2];
p2++;`enter code here`
}
}
cout << "Mark3" << endl;
delete [] array2;
cout << "Delete array2 " << endl;
delete [] array1;
cout << "Delete array1 " << endl;
}

Answer 1

The syntax
new int(n+1)
Creates a single int on the free-store and initialises it with n+1, and right away you access it out of bounds with array[1]. You want brackets:
new int[n + 1]
Which will create an array. The same goes for every other place like that in the program.
Also, since you are starting your loop at 1, the object array[0] is uninitialised and you get undefined behaviour if you access it, which you do. This is wasting an array element for nothing and setting up traps for yourself, I recommend you don't add 1 to the array size and start your indices from 0.