running_sum.rb

# @param {Integer[]} nums
# @return {Integer[]}
def running_sum(nums)
    nums.each_with_index do |item, index|
       if index == 0
           next
       end
        nums[index] = nums[index] + nums[index-1]
    end
end

Cleaner approach:

def running_sum(nums)
  for index in (1..nums.size-1) do
    nums[index] += nums[index-1]  
  end
  nums
end