Merge sort C++ not sorting

Question

I am doing the Algorithm Part I course, and I implemented merge sort using C++. It was supposed to do the in-place sorting but It doesn't sort the array. I am not able to find the reason. Hoping to get a positive response. Thank you.

void merge(int *a, int *aux, int low, int mid, int high) {
    for (int k = low; k <= high; ++k)
        aux[k] = a[k]; // copy
    int i = low, j = mid + 1;
    for (int k = low; k <= high; ++k) { // merge
        if (i > mid)
            a[k] = aux[j++];
        else if (j > mid)
            a[k] = aux[i++];
        else if (aux[i] > aux[j])
            a[k] = aux[j++];
        else
            a[k] = aux[i++];
    }
}

void sort(int *a, int *aux, int low, int high) {
    if (high <= low)
        return;
    int mid = low + (high - low) / 2;
    sort(a, aux, low, mid);
    sort(a, aux, mid + 1, high);
    merge(a, aux, low, mid, high);
}

void sort(int *a, int N) {
    int *aux = new int[N];
    sort(a, aux, 0, N - 1);
}

Answer 1

Your merge function is logically incorrect. For any iteration the last two conditions will never be executed. j will be always greater than mid , control will not reach beyond the second expression.

Modify your definition of merge function to something like this:

void merge(int *a, int *aux, int low, int mid, int high) {
    for (int k = low; k <= high; ++k)
        aux[k] = a[k]; // copy
    int i = low, j = mid + 1;
    for (int k = low; k <= high; ++k) { // merge

// changes begin here

        if (i <= mid and j <= high) {
            if (aux[i] > aux[j])
                a[k] = aux[j++];
            else
                a[k] = aux[i++];
        }
        else if (i > mid)
            a[k] = aux[j++];
        else // if (j > high)
            a[k] = aux[i++];

// changes end here

    }
}