Number of Atoms

A chemical formula string represents different elements and their quantities.
Each element begins with a capital letter and may include lowercase letters after it.
A number following an element indicates how many times that element appears—if no number is present, the quantity is 1.

Formulas may contain:

• Simple sequences, like "NaCl"

• Repeated groups, like "H2O" or "C6H12O6"

• Nested parentheses, e.g. "(OH)2" or "K4(ON(SO3)2)2"

Your task is to compute how many times each atom appears in the whole formula.
Finally, return a string that lists all atom names in sorted order, and append their counts (except when the count is 1).

All final quantities will fit in a 32-bit integer.