How to recover the internal nodes of a Newick tree in a dictionary?

Question

I have a following Newick tree : ((((A,B)1,C)2,((((D,E)3,F)4,G)5,(((((H,I)6,J)7,K)8,L)9,M)10)11)12,N)13;"

where the letters are the leaves and the numbers the in