150. Evaluate Reverse Polish Notation(Leetcode每日一题-2021.03.20)

最新推荐文章于 2022-03-10 05:23:35 发布

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最新推荐文章于 2022-03-10 05:23:35 发布

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Problem

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, and /. Each operand may be an integer or another expression.

Note that division between two integers should truncate toward zero.

It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.

Constraints:

1 <= tokens.length <= 10^4
tokens[i] is either an operator: “+”, “-”, “*”, or “/”, or an integer in the range [-200, 200].

Example1

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

Example2

Input: tokens = [“4”,“13”,“5”,"/","+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example3

Input: tokens = [“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

Solution

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        int ret = 0;
        stack<int> stk;
        for(int i = 0;i<tokens.size();++i)
        {
            string token = tokens[i];
            if(token != "+" && token != "-" && token != "*" && token != "/")
            {
                stk.push(stoi(token));
            }
            else
            {
                int op1 = stk.top();
                stk.pop();

                int op2 = stk.top();
                stk.pop();
                if(token == "+")
                    ret = op1 + op2;
                else if(token == "*")
                    ret = op1 * op2;
                else if(token == "-")
                    ret = op2 - op1;
                else 
                    ret = op2 / op1;

                stk.push(ret);
            }
        }

        ret = stk.top();
        stk.pop();

        return ret;

    }
};