def target_distribution(self, samples):
        """posterior probability
        
        Parameters
        ----------
        samples: list
            a collection of sample, it's a (NUM_OF_INSTANCE * NUM_OF_FUNCTIONS) matrix,
            representing{{w11, w12, ..., w1k}, {w21, w22, ... w2k}, ...{wk1, wk2,..., wkk}}
        
        Returns
        -------
        float
            posterior probability
        """
        curr_likelihood = self.likelihood(samples)
        curr_prior = self.prior(samples)
        ret = np.ones(NUM_OF_INSTANCE)
        for i in range(NUM_OF_INSTANCE):
            ret[i] = curr_likelihood[i] * curr_prior[i]
        return ret
posterior probability
        
        Parameters
        ----------
        samples: list
            a collection of sample, it's a (NUM_OF_INSTANCE * NUM_OF_FUNCTIONS) matrix,
            representing{{w11, w12, ..., w1k}, {w21, w22, ... w2k}, ...{wk1, wk2,..., wkk}}
        
        Returns
        -------
        float
            posterior probability