Why almost sure convergence cannot imply convergence in $L^p$?

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Almost sure convergence cannot imply convergence in $L^p$. However I can't find a rigorous counterexample?

Can anybody help me? Any helpful ideas would be greatly appreciated!

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Take the unit interval with Lebesgue measure and $f_n(x)=n^\alpha\mathbf{1}_{(0,1/n)}$, where $\alpha>0$. Whatever $\alpha$ is, this sequence converges almost everywhere to $0$. Can you find $\alpha$ such that the convergence in $\mathbb L^p$ does not hold?