150. 逆波兰表达式求值(Medium)

题目描述

求出表达式的值，根据 逆波兰表达式法

样例

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

题目解析

经典栈

代码

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack = []
        for char in tokens:
            if  char in '+-*/':
                x2 = stack.pop()
                x1 = stack.pop()
                if char == '+': stack.append(x1 + x2)
                if char == '-': stack.append(x1 - x2)
                if char == '*': stack.append(x1 * x2)
                if char == '/': stack.append(int(x1 / x2))
            else:
                stack.append(int(char))
        return stack[0]

Go 的使用

func Pop(nums *[]int) int {
    n := len(*nums)
    value := (*nums)[n - 1] // 一定要加 括号
    *nums = (*nums)[:n - 1] 
    return value
}

func evalRPN(tokens []string) int {
    n := len(tokens)
    stack := make([]int, 0)
    for i:= 0;i < n; i++ {
        if strings.Contains("+-*/", tokens[i]) {
            v1 := Pop(&stack)
            v2 := Pop(&stack)
            if tokens[i] == "+" {
                stack = append(stack, v1 + v2)
            } else if tokens[i] == "-" {
                stack = append(stack, v2 - v1)
            } else if tokens[i] == "*" {
                stack = append(stack, v1 * v2)
            } else {
                stack = append(stack, v2 / v1)
            }
        } else {
            tokenInt, _ := strconv.Atoi(tokens[i]) // value, error = strconv.Atoi
            stack = append(stack, tokenInt)
        }
    }  
    return stack[0]
}

func Pop(nums *[]int) int {
    n := len(*nums)
    value := (*nums)[n - 1]
    *nums = (*nums)[:n - 1]
    return value
}

func evalRPN(tokens []string) int {
    n := len(tokens)
    stack := make([]int, 0)
    for i:=0; i < n; i++ {
        if strings.Contains("+-*/", tokens[i]){
            v1 := Pop(&stack)
            v2 := Pop(&stack)
            if tokens[i] == "+" {
                stack = append(stack, v1 + v2) 
            } else if tokens[i] == "-" {
                stack = append(stack, v2 - v1) 
            } else if tokens[i] == "*" {
                stack = append(stack, v1 * v2) 
            } else {
                stack = append(stack, v2 / v1) 
            }
        } else {
	        value, _ := strconv.Atoi(tokens[i])
	        stack = append(stack, value)
        }
    }
    return stack[0]
}