  A complexity-adaptive tree search algorithm is proposed for
$\boldsymbol{G}_N$-coset codes that implements maximum-likelihood (ML) decoding
by using a successive decoding schedule. The average complexity is close to
that of the successive cancellation (SC) decoding for practical error rates
when applied to polar codes and short Reed-Muller (RM) codes, e.g., block
lengths up to $N=128$. By modifying the algorithm to limit the worst-case
complexity, one obtains a near-ML decoder for longer RM codes and their
subcodes. Unlike other bit-flip decoders, no outer code is needed to terminate
decoding. The algorithm can thus be applied to modified
$\boldsymbol{G}_N$-coset code constructions with dynamic frozen bits. One
advantage over sequential decoders is that there is no need to optimize a
separate parameter.
