And finally, let's implement the flatten function in terms of reduce:

func flattenIntermsOfReduce<Element>(elements: [[Element]]) -> [Element] { 
    return elements.reduce([]) { $0 + $1 } 
} 

let flattened = flattenIntermsOfReduce(elements: aTwoDimArrayOfNumbers) 

flattened will be [1, 3, 5, 2, 4, 6].