CareerCup

int[] retArray SumColumns(int[] array, int numRows){
int n = array.Length;
if (n % numRows != 0)
  throw new Exception("Invalid parameters");

 int columns = n / numRows;
 int[] result = new int[columns];
 
 for (int i = 0 ; i < columns; ++i) 
  for (int j = 0; j < numRows; ++j)
   result[i] += array[i + columns * j];

 return result;
}

Time Complexity: O(columns * numRows) = O(n);
Space Complexity: O(columns) - the result array

The simplest and correct solution.
Let there be 'n' elements in single dimentional array 'A'.
Let the no of columns be 'noOfColumns'. In this question, noOfColumns = 2.

for(i=0;i<n;i++)
{
    int column = i%noOfColumn;
    result[column]+=A[i];
}

Complexity - 0(n)

an example of numrows=2 is the following array or u can call it a matrix:

3 4 7 2
2 6 0 9
-------
5 10 7 11 -> resultant one D array returned by the subroutine.

Given k rows and an array of size n; we want to add every (n/k)th element of the array

for every element 'i' in array
        array[i%k] += array[i]
// not the first k elements of the array is the required output
-> Time: O(n), Space: O(1)

int* ArrayColSum (int*pAray, int numRow, int size)
{

int numOfCol = size/numRow;
int* pColSum = (int*)malloc (numOfCol * sizeof (int));
int i = 0;
int j = 0;
int count = 0;
memset (pColSum, 0x00, numOfCol);
while (j < numOfCol)
{
while (i < size)
{
pColSum [j] += pAray [i];
i = i + numOfCol;
}
j++;
i = j;
}
return pColSum;
}

complexity = (size/numOfCol)*numOfCol = size
O(size)

int[] retArray SumColumns(int[] array, int numRows)
{
   // you need array length also
   int numCols = len(array)/numRows;
   int *sum = malloc(sizeof(int)*numCols);
   int count = 0;
   for(int i = 0; i < len; i++)
   {

      int col = i%numCols;
      sum[col] = a[i]
   }
   return sum;
}

public static int[] retArray(int[] arr, int numR)
{
int [] ret = new int[arr.Length/numR];
int addon = arr.Length / numR;
int k =0;
for (int i = 0; i < arr.Length; i++)
{
Console.WriteLine(k % addon);
ret[k % addon] = ret[k % addon] + arr[i];
k++;
}
return ret;
}

public int[] SumOfCols(int[] a, int rows)
        {
            if (rows < 2 || (a.Length % rows != 0))
                throw new Exception("invalid parameters");
            int cols = a.Length / rows;
            int[] result = new int[cols];

            for (int i = 0; i < a.Length; i++)
            {
                int index = i % cols;
                result[index] += a[i];
            }
            return result;
        }