code: public static int[] getInt2DColumnSum(int[][] apsp) {
          int[] colSum = new int[apsp.length];
          int sum;
          for (int i = 0; i < apsp.length; i++) {
              sum = 0;
              for (int j = 0; j < apsp.length; j++) {
                  sum += apsp[i][j];
              }
              colSum[i] = sum;
          }
          return colSum;
      }
docs: Sums up the columns in a 2D int matrix.
  
  @param apsp The 2D int matrix
  @return A 1D matrix containing the column sum of the 2D matrix