# 150. Evaluate Reverse Polish Notation

Medium
Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are `+`, `-`, `*`, and `/`. Each operand may be an integer or another expression.
Note that division between two integers should truncate toward zero.
It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.
Example 1:
Input: tokens = ["2","1","+","3","*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9
Example 2:
Input: tokens = ["4","13","5","/","+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6
Example 3:
Input: tokens = ["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
Constraints:
• `1 <= tokens.length <= 10^4`
• `tokens[i]` is either an operator: `"+"`, `"-"`, `"*"`, or `"/"`, or an integer in the range `[-200, 200]`.

### 解題

O(n)
Runtime: 0 ms, faster than 100%
Memory Usage: 4.5 MB, less than 92%
func evalRPN(tokens []string) int {
stack := make([]int ,0)
for i := 0; i < len(tokens); i++ {
if tokens[i] == "+" {
num1 := stack[len(stack) - 1]
num2 := stack[len(stack) - 2]
stack = stack[:len(stack) - 2]
stack = append(stack, num1 + num2)
} else if tokens[i] == "-" {
num1 := stack[len(stack) - 1]
num2 := stack[len(stack) - 2]
stack = stack[:len(stack) - 2]
stack = append(stack, num2 - num1)
} else if tokens[i] == "*" {
num1 := stack[len(stack) - 1]
num2 := stack[len(stack) - 2]
stack = stack[:len(stack) - 2]
stack = append(stack, num1 * num2)
} else if tokens[i] == "/" {
num1 := stack[len(stack) - 1]
num2 := stack[len(stack) - 2]
stack = stack[:len(stack) - 2]
stack = append(stack, num2 / num1)
} else {
if s, err := strconv.Atoi(tokens[i]); err == nil {
stack = append(stack, s)
}
}
}
return stack
}