Complexity of recursion T(n) = 2T(n-1) + C?
The exact equation is (if $T(1) = 1$):

$$ T(n) = 2T(n-1) + 1 = 2(2T(n-2) + 1) + 1 = $$
$$ 2^2 T(n-2) + 2 + 1 = $$
$$ 2^2(2T(n-3) + 1) + 2 +‌ 1 = 2^3 T(n-3) + 2^2 + 2 + 1$$

Hence, $T(n) = 2^{n-1} + 2^{n-2} + \cdots + 1 = 2^n - 1$ (by mathematical induction).