j is dependent on i, therefore unroll the dependency, means analyze for i only
if i=1 ----> inner loop executes log(1) times
if i=2 ----> inner loop executes log(2) times
if i=3 ----> inner loop executes log(3) times
.
.
if i=n ----> inner loop executes log(n) times.
combine them ==> log(1)+log(2)+.....+log(n) = log ( 1.2.3...n ) = log ( n! ) = n log(n)