Leetcode 770. Basic Calculator IV

Parse and evaluate a space-separated arithmetic expression with integers, multi-letter variables, parentheses and +, -, * (obeying precedence), applying given variable substitutions, then expand and simplify into polynomial-like terms by multiplying out, combining like terms (summing coefficients), dropping zero coefficients, and returning terms sorted by descending degree and lexicographic order. Key challenges: implement a parser/evaluator with correct precedence and represent terms as (coefficient, sorted variable multiset) to support multiplication, hashing, and ordering.