150. Evaluate Reverse Polish Notation

题目

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

题目大意

计算逆波兰表达式。

解题思路

这道题就是经典的考察栈的知识的题目。

代码

package leetcode

import (
	"strconv"
)

func evalRPN(tokens []string) int {
	stack := make([]int, 0, len(tokens))
	for _, token := range tokens {
		v, err := strconv.Atoi(token)
		if err == nil {
			stack = append(stack, v)
		} else {
			num1, num2 := stack[len(stack)-2], stack[len(stack)-1]
			stack = stack[:len(stack)-2]
			switch token {
			case "+":
				stack = append(stack, num1+num2)
			case "-":
				stack = append(stack, num1-num2)
			case "*":
				stack = append(stack, num1*num2)
			case "/":
				stack = append(stack, num1/num2)
			}
		}
	}
	return stack[0]
}

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