Evaluate Reverse Polish Notation

In reverse Polish notation (RPN), an operator comes after its two operands. For example, 3 4 + means 3 + 4, and 3 4 + 2 * means (3 + 4) * 2. No brackets are needed.

You get the expression as an array of string tokens. Return its value as a number.

• Tokens are integers, or one of the operators "+", "-", "*", "/". Integers may be negative, like "-4".
• For a b -, the result is a - b. For a b /, it is a / b. The first operand is the one written first.
• Dividing keeps only the whole-number part and rounds toward zero: 7 / -2 is -3, not -4.
• You can trust the input. It is a well-formed expression with at least one token, and no division ever has 0 on the right.