Project Euler #18 in Haskell

Project Euler #18 in Haskell

I am currently learning functional programming and Haskell coming from a
python background. To help me learn, I decided to do some project euler
problems (http://projecteuler.net/problem=18). Currently I am on #18.
Starting with this string,
"75
95 64
17 47 82
18 35 87 10
20 04 82 47 65
19 01 23 75 03 34
88 02 77 73 07 63 67
99 65 04 28 06 16 70 92
41 41 26 56 83 40 80 70 33
41 48 72 33 47 32 37 16 94 29
53 71 44 65 25 43 91 52 97 51 14
70 11 33 28 77 73 17 78 39 68 17 57
91 71 52 38 17 14 91 43 58 50 27 29 48
63 66 04 68 89 53 67 30 73 16 69 87 40 31
04 62 98 27 23 09 70 98 73 93 38 53 60 04 23"
I managed to convert it into a nested array using this function:
map (\x -> map (\y -> read y :: Int) (words x)) (lines a)
That function outputs this:
[[75],[95,64],[17,47,82],[18,35,87,10],[20,4,82,47,65],[19,1,23,75,3,34],[88,2,77,73,7,63,67],[99,65,4,28,6,16,70,92],[41,41,26,56,83,40,80,70,33],[41,48,72,33,47,32,37,16,94,29],[53,71,44,65,25,43,91,52,97,51,14],[70,11,33,28,77,73,17,78,39,68,17,57],[91,71,52,38,17,14,91,43,58,50,27,29,48],[63,66,4,68,89,53,67,30,73,16,69,87,40,31],[4,62,98,27,23,9,70,98,73,93,38,53,60,4,23]]
Is there a way to write that conversion function without the lambdas and
make it more readable?
How would I convert this nested array to a tree structure defined like so:
data Tree a = EmptyTree | Node a (Tree a) (Tree a) deriving (Show, Read, Eq)
Or would it just be easier to leave it as an array and solve it from there?