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All atom names consist of lowercase letters, except for the first character which is uppercase.
Three-letter element abbreviations such as Uue are possible.
Finally, your add_up() function did not need to be written — you've just reinvented the addition method of collections.Counter.
Furthermore, your add_up() function did not need to be written — you've just reinvented the addition method of collections.Counter.
Strictly speaking, your regex "([A-Z][a-z]?)" makes an assumption that does not meet the specification:
All atom names consist of lowercase letters, except for the first character which is uppercase.
Three-letter element abbreviations such as Uue are possible.
Finally, your add_up() function did not need to be written — you've just reinvented the addition method of collections.Counter.
You have a couple of good ideas:
Unfortunately, those ideas were ineffectively applied, such that the main code is still very complicated and difficult to follow.
Iterating through the formula character by character (using your while i < len(formula) loop) defeats the purpose of using regular expressions. Furthermore, you should not need tests like c == "(", c == ")", and .isdigit().
Rather, the parsing should be mainly driven by re.finditer(), using one regular expression that is constructed to classify everything that it can encounter:
Each of those tokens should have a named capture group to help you figure out what was matched.
Furthermore, your add_up() function did not need to be written — you've just reinvented the addition method of collections.Counter.
Unfortunately, LeetCode expects the solution to be packaged inside a weird Solution class that is not really a class, but a namespace. (It calls the countOfAtoms() "method" even though there is no meaningful constructor.) I've decided to tweak it into a @classmethod instead.
from collections import Counter import re class Solution(object): RE = re.compile( r'(?P<atom>[A-Z][a-z]*)(?P<atom_count>\d*)|' r'(?P<new_group>\()|' r'\)(?P<group_count>\d*)|' r'(?P<UNEXPECTED_CHARACTER_IN_FORMULA>.+)' ) @classmethod def atom_count(cls, stack, atom, atom_count='', **_): """Handle an atom with an optional count, e.g. H or Mg2""" stack[-1][atom] += (1 if atom_count == '' else int(atom_count)) @classmethod def new_group(cls, stack, **_): """Handle an opening parenthesis""" stack.append(Counter()) @classmethod def group_count(cls, stack, group_count='', **_): """Handle a closing parenthesis with an optional group count""" group_count = 1 if group_count == '' else int(group_count) group = stack.pop() for atom in group: group[atom] *= group_count stack[-1] += group @classmethod def countOfAtoms(cls, formula): stack = [] cls.new_group(stack) for m in cls.RE.finditer(formula): getattr(cls, m.lastgroup)(stack, **m.groupdict()) return ''.join( atom + (str(count) if count > 1 else '') for atom, count in sorted(stack.pop().items()) )