Given a mathematical expression in infix notation, evaluate its result.
For example:
3 * (2 + 5) = 21
3 * (2 + 5 * 2) = 36
Tip: In the usual infix notation, operators are written between the operands. In the postfix notation, the operators are written after the operands.
Infix: 3 * (2 + 5)
Postfix: 3 2 5 + *
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