103. Basic Calculator - Expression Evaluator

You're building a calculator app that needs to evaluate mathematical expressions. The calculator should handle addition, subtraction, parentheses, and spaces, but no multiplication or division.

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().

Requirements

1. Basic Functionality

• Evaluate expressions with + and - operators
• Handle parentheses for grouping
• Handle spaces (ignore them)
• Support negative numbers
• Return integer result

Example Usage

calculate("1 + 1");
// 2

calculate(" 2-1 + 2 ");
// 3

calculate("(1+(4+5+2)-3)+(6+8)");
// 23

calculate("-2+ 1");
// -1

Real-World Context

This problem models real calculator features:

• Calculator apps: Evaluate user input expressions
• Spreadsheet formulas: Parse and evaluate formulas
• Expression parsers: Build parsers for mathematical expressions
• Code evaluators: Evaluate mathematical code expressions