## Question

You are given an array of strings `tokens`

that represents an arithmetic expression in a Reverse Polish Notation.

Evaluate the expression. Return *an integer that represents the value of the expression*.

**Note** that:

- The valid operators are
`'+'`

,`'-'`

,`'*'`

, and`'/'`

. - Each operand may be an integer or another expression.
- The division between two integers always
**truncates toward zero**. - There will not be any division by zero.
- The input represents a valid arithmetic expression in a reverse polish notation.
- The answer and all the intermediate calculations can be represented in a
**32-bit**integer.

**Example 1:**

```
Input: tokens = ["2","1","+","3","*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9
```

## Algorithm

This question is a typical application of stack.

For expression calculation, we have Polish Notation and Reverse Polish Notation.

Normally we will need a stack to store latest calculation results. If we have met the operator, we could get the two operands from stack pop.

## Code

class Solution { public int evalRPN(String[] tokens) { Stack<Integer> stack = new Stack(); for (int i = 0; i < tokens.length; i++) { if ("+".equals(tokens[i])) { stack.push(stack.pop() + stack.pop()); } else if ("-".equals(tokens[i])) { int num1 = stack.pop(); int num2 = stack.pop(); stack.push(num2 - num1); } else if ("*".equals(tokens[i])) { stack.push(stack.pop() * stack.pop()); } else if ("/".equals(tokens[i])) { int num1 = stack.pop(); int num2 = stack.pop(); stack.push(num2 / num1); } else { stack.push(Integer.valueOf(tokens[i])); } } return stack.pop(); } }