Expected Number of flips for alternating Heads/Tails 10 times What is the expected number of flips needed to flip a coin 10 times and have the outcome be alternating heads/tails (starting with heads, then tails, then heads etc...). 
I wrote a c++ program and it gives me 2,730. Is this correct and how would you do this mathematically?
 A: Let $x$ be the expected number of flips to get $(HT)^5$ and $y$ the expected number of further tosses required if we are starting with $XH$ where we cannot use part of $X$ to form the first part of the required run (eg $X$ is empty or ends in $H$).
The first toss is $H$ or $T$, so $x=\frac{1}{2}(1+y)+\frac{1}{2}(1+x)$. Hence $y=x-2$.
How consider the outcomes: $T,HH,(HT)T,(HT)HH,(HT)^2T,(HT)^2HH,\dots,(HT)^4T,(HT)^4HT,(HT)^4HH,(HT)^5$.
They give $$x=\frac{1}{2}(x+1)+\frac{1}{2^2}(y+2)+\frac{1}{2^3}(x+3)+\dots+\frac{1}{2^9}(x+9)+\frac{1}{2^{10}}(y+10)+\frac{1}{2^{10}}10$$ $$=\frac{1}{2}(x+1)+\frac{1}{2^2}x+\frac{1}{2^3}(x+3)+\dots+\frac{1}{2^9}(x+9)+\frac{1}{2^{10}}(x+8)+\frac{1}{2^{10}}10$$ $$=x\left(1-\frac{1}{1024}\right)+\left(\frac{1}{2}+\frac{3}{2^3}+\frac{2}{2^4}+\frac{5}{2^5}+\frac{4}{2^6}+\frac{7}{2^7}+\frac{6}{2^8}+\frac{9}{2^9}+\frac{18}{2^{10}}\right)$$ $$=x\left(1-\frac{1}{1024}\right)\frac{341}{256}$$ So finally $x=1364$.
$\textbf{Comments}$
This is easy to get wrong. I got it wrong, but fortunately @Did picked up the error (see comment below).
The video the OP refers to seems to be about 10 consecutive heads. That case is different because a single $T$ wrecks the run and puts you back to the start. In that case you can consider the outcomes $T,HT,HHT,HHHT,\dots,HHHHHHHHHT,HHHHHHHHHH$. The all but the last (which is a run of 10 heads) put you back to the start. So we have $$x=\frac{1}{2}(x+1)+\frac{1}{4}(x+2)+\dots+\frac{1}{1024}(x+10)+\frac{10}{1024}$$ giving $$\frac{1}{1024}x=\frac{1}{2}+2\left(\frac{1}{2}\right)^2+3\left(\frac{1}{2}\right)^3+\dots+10\left(\frac{1}{2}\right)^{10}+\frac{10}{1024}$$ giving $$x=2046$$
In both cases it is not too difficult to generalise to the case of $n$ flips.
Note that a run of 10 heads or a run of 10 tails are the hardest runs to get because a single wrong flip puts you back to the start, whereas if you are aiming for alternate heads and tails, an unwanted $H$ knocks out your run but gives the first element of a new run. So the 2730 from the $C^{++}$ must be some kind of error.
