Leetcode - Basic Calculator

Question

Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.

Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

Ideas:

• This is Leetcode - Basic Calculator II with the twist that the input string can include ()
• Whenever we encounter parens, we enter into a BCII sub problem
• On (
  • Save current state (stack, lastOp)
  • Reset state for subproblem
• On )
  • Push currNum to stack based on lastOp rule
  • val is sum of stack
  • set val to CurrNum since this is the inner expression evaluated
  • Pop last state from context and revive stack, lastOp
• There is one edge case for unary ‘-’ where if there is no currNum, just pretend that currNum is 0

Solution