shuffle_array.rb

# @param {Integer[]} nums
# @param {Integer} n
# @return {Integer[]}
def shuffle(nums, n)
    result = []
    first = 0
    second = n
    index = 0
    while result.size < 2*n
      result[index] = nums[first]
      first += 1
      index += 1
      result[index] = nums[second]
      second += 1
      index += 1
    end
    result
end

nums = [2,5,1,3,4,7] 
n = 3
p shuffle(nums, n)

Use two pointers to create the new array of 2n elements. The first starting at the beginning and the other starting at (n+1)th position. Alternate between them and create the new array.

Find the relationship between the given input parameters. This relationship is used in the terminating condition of the while loop.