one-line solution

[fsferrara]

thanks to Marco Sciamanna... my first solution was actually a bubble-sort 👎

[msciamanna]

The idea was to put on left the highest digits.

So:

1. Turn every element of the array (integer) in its string representation.
2. Sort the array elements (strings) in reverse alphabetical order
3. Concatenate the array elements
4. Cast the result (string) into an integer

The instructions:

Step	Instruction	Result
0	sourceArray = [10, 51, 17, 8, 75, 9] // input array	[10, 51, 17, 8, 75, 9]
1	sourceArray.map{String($0)}	["10", "51", "17", "8", "75", "9"]
2	sourceArray.map{String($0)}.sort(>)	["9", "8", "75", "51", "17", "10"]
3	sourceArray.map{String($0)}.sort(>).joinWithSeparator("")	"9875511710"
4	Int(sourceArray.map{String($0)}.sort(>).joinWithSeparator(""))!	9875511710

[msciamanna]

I realised that it doesn't work for some particular inputs :(.

Input array: [ ..., n, ..., nm, ... ] where n > m.

The algorithm sorts the array this way:

[ ..., "nm", "n", ... ], because "nm" > "n"

and then produces the output:

...nm n... that is wrong, because ...n nm... would have been higher.

Example. Using the input array: [9, 90]

The script gives 909 (because "90" > "9" in the alphabetical order) instead of the expected result 990.

[fsferrara]

As you have pointed out, a lexicographic sort and concatenation is close, but not quite correct. For example, for the numbers 5, 54, and 56 a lexicographic sort will produce {5, 54, 56} (in increasing order) or {56, 54, 5} (in decreasing order), but what we really want is {56, 5, 54}, since that produces the largest number possible.