Evaluate a Nested Math Expression

You are given a string s representing a nested arithmetic expression.
The expression uses function-call syntax:
add(x, y) evaluates to x + y.
sub(x, y) evaluates to x - y.
Each argument x or y is either an integer or another nested add/sub expression. The expression is guaranteed to be syntactically valid.
Return the integer value of the expression.

Function
evaluateExpression(s: String) → int

Examples
Example 1
s = "add(1,sub(1,0))"
return = 2
sub(1,0) = 1, then add(1,1) = 2.
Example 2
s = "add(sub(5,2),sub(1,4))"
return = 0
sub(5,2) = 3 and sub(1,4) = -3, so the result is 0.
Example 3
s = "sub(add(7,8),sub(3,1))"
return = 13

Constraints
1 <= s.length <= 2 * 10^5
Integer literals fit in a 32-bit signed integer.
The input expression is valid and contains only add, sub, integer literals, parentheses, commas, and optional spaces.
The final result fits in a 32-bit signed integer.

The trick is to treat this as a stack-based evaluation, not a grammar problem. Scan left to right. When you hit a function name, push an operator marker. When you hit a number, push it as a value. On a closing parenthesis, pop the two operands and the operator, compute, and push the result back. A comma and spaces just get skipped. The common pitfall is recursive descent in a language with a shallow recursion limit, since 2 * 10^5 characters can nest tens of thousands of levels. The second pitfall is negative numbers. Intermediate results can go negative, and literals may carry a minus sign, so parse the sign with the digits. Watch operand order on sub, because x minus y isn't commutative and a stack pops in reverse. If the live OA has you fumbling that order, StealthCoder is the hedge that keeps you moving.

StealthCoder is the hedge for the one pattern you didn't drill. It runs invisibly during the screen share.