150. 逆波兰表达式求值

原题

https://leetcode.cn/problems/evaluate-reverse-polish-notation/description/

思路

栈

复杂度

时间：O(n)
空间：O(n)

Python代码

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack = []
        for token in tokens:
            if token in ['+', '-', '*', '/']:
                y =  stack.pop()
                x = stack.pop()
                if token == '+':
                    stack.append(x + y)
                elif token == '-':
                    stack.append(x - y)
                elif token == '*':
                    stack.append(x * y)
                else:
                    d, m = divmod(x, y)
                    if d >= 0:
                        stack.append(d)
                    elif d < 0 and m == 0:
                        stack.append(d)
                    else:
                        stack.append(d + 1)
                    
            else:
                stack.append(int(token))
        return stack[0]

        

Go代码

func evalRPN(tokens []string) int {
	stack := []int{}
	for _, token := range tokens {
		if token == "+" || token == "-" || token == "*" || token == "/" {
			y := stack[len(stack)-1]
			x := stack[len(stack)-2]
			stack = stack[:len(stack)-2]
			if token == "+" {
				stack = append(stack, x+y)
			} else if token == "-" {
				stack = append(stack, x-y)
			} else if token == "*" {
				stack = append(stack, x*y)
			} else {
                stack = append(stack, x/y)
			}
		} else {
			i, err := strconv.Atoi(token)
			if err != nil {
				fmt.Println("error: ", err)
			}
			stack = append(stack, i)
		}
	}
	return stack[0]
}