There are three towers named as Left, Middle, Right.
Tower Left contains 3 discs in decreasing size from bottom to top.
We have to move these 3 discsr.
step 1: Move 2 discs from . $n-1$ discs
Step 2: Move the remaining disc from $1-movement$
Step 3: Move 2 discs from . $n-1$ discs
$T(n) = T(n-1) + T(n-1) + 1$
$T(n) = 2T(n-1) + 1$