Bioinformatics Contest 2017 (task1)

Everybody knows that a huge number of different chemical reactions happen in cells. A reaction takes as an input a set of chemicals (substrates) and converts them to another set (products). Here we consider all reactions to be one-directional. A substrates for a reaction could be chemicals from the environment or products from other reactions. 
A scientist John Doe was given a cell and a list of chemicals that are present in the environment at the beginning. He already knows what reactions could happen in the cell. You should help him to understand which chemicals could appear in the cell.

Input Format

The first line contains initial chemicals separated by spaces. There is always at least one initial chemical. Each chemical is represented by a random positive integer that fits in 4-byte integer type. Each of the following lines describes a reaction, one per line. Each reaction is presented as two lists of integers separated by '->': the list of chemicals, separated by '+', that is needed to perform a reaction and the list of chemicals, separated by '+', that are produced as a result of the reaction. Each chemical could be present in each reaction maximum 2 times: one time at the left part and the other time at the right part (for example, a catalyst could appear in both parts).
Constraints for the easy version: a total number of chemicals through all reactions does not exceed 103
.
Constraints for the hard version: a total number of chemicals through all reactions does not exceed 105.

import sys


def chem_recorder(data_set):
    while len(data_set) > 0:
        new_data_set = set()
        for add_chem in data_set:
            if add_chem not in chem_in_cell:
                chem_in_cell.add(add_chem)
                if add_chem in chems_lines:
                    counter = 0                    for chem_line in chems_lines[add_chem]:
                        chem_line[0].remove(add_chem)
                        if len(chem_line[0].difference(chem_in_cell)) == 0:
                            new_add = chem_line[1].difference(chem_in_cell)
                            new_data_set = new_data_set.union(new_add)
                            chems_lines[add_chem].pop(counter)
                        counter += 1        data_set = new_data_set.copy()


all_lines = sys.stdin.readlines()
in_cell = all_lines.pop(0)
chem_in_cell = set(in_cell.strip().split(' '))
xxx = set(in_cell.strip().split(' '))
chems_lines = {} # 'chem' => [[{left_line1}, {right_line1}], [{left_line2}, {right_line2}]]for line in all_lines:
    line = line.strip()
    l1 = line.split('->')
    left_line = set(l1[0].split('+'))
    right_line = set(l1[1].split('+'))
    not_in_cell_chem = left_line.difference(chem_in_cell)
    if len(not_in_cell_chem) > 0:
        for nic_chem in not_in_cell_chem:
            if nic_chem in chems_lines:
                flag = 0                for chem_line in chems_lines[nic_chem]:
                    if chem_line[0] == not_in_cell_chem:
                        chem_line[1] = chem_line[1].union(right_line)
                        flag = 1                        break                if flag == 0:
                    chems_lines[nic_chem].append([not_in_cell_chem, right_line])
            else:
                chems_lines[nic_chem] = [[not_in_cell_chem, right_line]]
    else:  # gote all chems        chem_recorder(right_line)
chem_recorder(xxx)
print(' '.join(chem_in_cell))