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Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open `(`

and closing parentheses `)`

, the plus `+`

or minus sign `-`

, **non-negative** integers and empty spaces ` `

.

You may assume that the given expression is always valid.

Some examples:

"1 + 1" = 2 " 2-1 + 2 " = 3 "(1+(4+5+2)-3)+(6+8)" = 23

**Note:** **Do not** use the `eval`

built-in library function.

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