#381 · #726 · Number of Atoms(原子的数量)

Given a chemical formula, return the count of each atom as a string in the form Name followed by its count (omitted when the count is 1), with names sorted alphabetically. Atoms start with an uppercase letter and may continue with lowercase letters; parentheses may be nested and followed by a multiplier. Constraints: 1 <= formula.length <= 1000.