Programming Que: Replace Elements in an Array to the Sum of all Num >=Element

This code snippet talks about basic and efficient way to replace elements in an array to the sum of all num >=element.

I remember, once I have been asked to replace elements in a sorted array to the sum of all elements which are greater or equal to that element.

Example

Input array A1 = {5, 12, 18, 22, 27}

Output Array should look be A1 = {84, 79, 67, 49, 27}

A[0] = 5+12+18+22+27 = 84

A[1] = 12+18+22+27 = 79

A[2] = 18+22+27 = 67

A[3] = 22+27 = 49

A[4] = 27

In a rush I figured out a way which takes two arrays and as follows.

public static int [] ReplaceArrayElementWithSum(int[] A1)
{
    int sum = 0;
    int[] A2 = new int[A1.Length];
    for (int i = 0; i<A1.Length; i++)
    {
        sum = sum + A1[i];
    }
    A2[0] = sum;
    for (int i = 0; i < A1.Length; i++)
    {
        if (i == 0)
        {
            A2[i] = sum;
        }
        else
        {
            A2[i] = A2[i - 1] - A1[i - 1];
        }
    }
    return A2;
}

Now let’s do the same thing in more efficient way.

public static int[] ReplaceArrayElementWithSum2(int[] A1)
{
    for (int i = A1.Length-2; i>=0; i--)
    {
       A1[i] = A1[i] + A1[i+1];
    }
    return A1;
}

Please let me know if you can think of any other/better way.