leetcode刷题笔记(python3)--150. Evaluate Reverse Polish Notation

150. 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

解题思路

func evalRPN(tokens []string) int {
	stack := []int{}
	for _, v := range tokens {

		if val, err := strconv.Atoi(v); err == nil {
			stack = append(stack, val)
		} else {
			if len(stack) < 2 {
				return 0
			}
            n1, n2 := stack[len(stack)-2], stack[len(stack)-1]
            fmt.Println(n1,n2,v)
            stack = stack[:len(stack)-2]
			n := 0
			switch v {
			case "+":
				n += n1 + n2
			case "-":
				n += n1 - n2
			case "*":
				n += n1 * n2
			case "/":
				n += n1 / n2
			default:
				return 0
			}
			stack = append(stack, n)
		}
	}
	return stack[len(stack)-1]   
}