# 150. Evaluate Reverse Polish Notation (Medium)

https://leetcode.com/problems/evaluate-reverse-polish-notation/

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are `+`, `-`, `*`, `/`. Each operand may be an integer or another expression.

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

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

Example 2:

```Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6
```

Example 3:

```Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
Output: 22
Explanation:
((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22
```

## Solutions

``````class Solution {

public int evalRPN(String[] tokens) {
if (tokens == null || tokens.length == 0) {
return 0;
}

Stack<Integer> stack = new Stack<>();
for (int i = 0; i < tokens.length; i++) {
if (tokens[i].equals("+")) {
int val1 = stack.pop();
int val2 = stack.pop();

stack.push(val2 + val1);

continue;
}

if (tokens[i].equals("-")) {
int val1 = stack.pop();
int val2 = stack.pop();

stack.push(val2 - val1);

continue;
}

if (tokens[i].equals("*")) {
int val1 = stack.pop();
int val2 = stack.pop();

stack.push(val2 * val1);

continue;
}

if (tokens[i].equals("/")) {
int val1 = stack.pop();
int val2 = stack.pop();

stack.push(val2 / val1);

continue;
}

stack.push(Integer.valueOf(tokens[i]));
}

return stack.pop();
}
}
``````