find_better_split()

def find_better_split(self, var_idx):
    x,y = self.x.values[self.idxs,var_idx], self.y[self.idxs]

    for i in range(self.n):
        lhs = x<=x[i]
        rhs = x>x[i]
        if rhs.sum()<self.min_leaf or lhs.sum()<self.min_leaf: continue
        lhs_std = y[lhs].std()
        rhs_std = y[rhs].std()
        curr_score = lhs_std*lhs.sum() + rhs_std*rhs.sum()
        if curr_score<self.score: 
            self.var_idx,self.score,self.split = var_idx,curr_score,x[i]