177. Basic Calculator
HardStack
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().
Examples
Input: (1+(4+5+2)-3)+(6+8)
Output: 23
Explanation: Evaluates nested parentheses to 23.
Constraints
- 1 <= s.length <= 3 * 10^5
- s consists of digits, '+', '-', '(', ')', and ' '.
- s represents a valid expression.
- '+' is not used as a unary operation.
- '-' could be used as a unary operation and in this case, it will not be used directly after a '+' or another '-'.
- There will be no two consecutive operators in the input.
- Every number and running calculation will fit in a signed 32-bit integer.
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