Evaluate Reverse Polish Notation

题目介绍

LeetCode 150. Evaluate Reverse Polish Notation。

复杂度

时间复杂度: O(n), 空间复杂度: O(n)

解题思路

class Solution {
    func evalRPN(_ tokens: [String]) -> Int {
        var stack: [Int] = []
        for token in tokens {
            if token == "+" || token == "-" || token == "*" || token == "/" {
                let second = stack.popLast()!
                let first = stack.popLast()!
                switch token {
                case "+":
                    stack.append(first + second)
                case "-":
                    stack.append(first - second)
                case "*":
                    stack.append(first * second)
                case "/":
                    stack.append(first / second)
                default:
                    break
                }
            } else {
                // 数字
                stack.append(Int(token)!)
            }
        }
        return stack.last!
    }
}

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