Largest Sum of k in a Row

nums holds whole numbers. Add up k consecutive numbers, meaning numbers that sit next to each other in the array. Return the largest such sum.

• k is a whole number from 1 up to the length of nums, so at least one group of k always exists.
• Numbers can be negative, so the answer can be negative too.
• nums has up to 100,000 elements, and k can be large. So aim for one pass over the array.