150.Evaluate Reverse Polish Notation

Tags: Medium Stack

Link: https://leetcode.com/problems/evaluate-reverse-polish-notation/

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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.

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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

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Answer:

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> stn;

        for(auto s:tokens) {
            if(s.size()>1 || isdigit(s[0])) stn.push(stoi(s));
            else {
                auto b=stn.top(); stn.pop();
                auto a=stn.top(); stn.pop();

                switch(s[0]) {
                    case '+': a+=b; break;
                    case '-': a-=b; break;
                    case '*': a*=b; break;
                    case '/': a/=b; break;
                }
                stn.push(a);
            }
        }

        return stn.top();
    }
};

解析：相当于计算器的计算过程

从string类型转为int，float，double类型的方式：