Reverse Polish calculator

In reverse Polish notation, an operator comes after its two numbers. 3 4 + means 3 + 4. 2 3 4 * + means 2 + (3 * 4).

Write calculate_rpn(tokens). It takes a list of strings. Each string is a whole number such as "12" or "-3", or one of the operators "+", "-", "*" and "/". It returns the result as an integer.

• Read the tokens from left to right. A number goes on a stack. An operator takes the top two numbers from the stack and puts the result back. The second number from the top is the left side: ["5", "2", "-"] means 5 - 2.
• Division gives a whole number. It cuts off the decimals toward zero: 7 / 2 gives 3, and -7 / 2 gives -3.
• The input is valid. A divisor is never 0. At the end, the stack has exactly one number.

Examples

Call	Result
calculate_rpn(['3', '4', '+'])	7
calculate_rpn(['2', '3', '4', '*', '+'])	14
calculate_rpn(['7', '2', '/'])	3