I've rearranged the selected answer, used pointers for arrays and user input for number count is not pre-defined.
#include <iostream>
using namespace std;
void merge(int*, int*, int, int, int);
void mergesort(int *a, int*b, int start, int end) {
int halfpoint;
if (start < end) {
halfpoint = (start + end) / 2;
mergesort(a, b, start, halfpoint);
mergesort(a, b, halfpoint + 1, end);
merge(a, b, start, halfpoint, end);
}
}
void merge(int *a, int *b, int start, int halfpoint, int end) {
int h, i, j, k;
h = start;
i = start;
j = halfpoint + 1;
while ((h <= halfpoint) && (j <= end)) {
if (a[h] <= a[j]) {
b[i] = a[h];
h++;
} else {
b[i] = a[j];
j++;
}
i++;
}
if (h > halfpoint) {
for (k = j; k <= end; k++) {
b[i] = a[k];
i++;
}
} else {
for (k = h; k <= halfpoint; k++) {
b[i] = a[k];
i++;
}
}
// Write the final sorted array to our original one
for (k = start; k <= end; k++) {
a[k] = b[k];
}
}
int main(int argc, char** argv) {
int num;
cout << "How many numbers do you want to sort: ";
cin >> num;
int a[num];
int b[num];
for (int i = 0; i < num; i++) {
cout << (i + 1) << ": ";
cin >> a[i];
}
// Start merge sort
mergesort(a, b, 0, num - 1);
// Print the sorted array
cout << endl;
for (int i = 0; i < num; i++) {
cout << a[i] << " ";
}
cout << endl;
return 0;
}