Top-Down Mergesort Analysis
lTsort(1) = 1;
lTsort(n) = 2 Tsort(n/2) + en
lSubstituting
T (n) = 2 T(n/2) + en
     = 2 (T(n/4) + en/2) + en
     = 2 (2(T(n/8) +en/4) + en/2) + en
    = É
        = 2log(n)*1 + log(n)*en
        = n + en log(n)
ë  O(nlog(n))