LeetCode150：Evaluate Reverse Polish Notation

题目：

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

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

Some examples:
  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) –> 6

解题思路：

很简单的一题，直接利用栈实现，不多说了

实现代码：

#include <iostream>

#include <stack>

#include <vector>

#include <string>

#include <cstdlib> 

using namespace std;



/*

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



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



Some examples:

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9

  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

*/ 

class Solution {

public:

    int evalRPN(vector<string> &tokens) {

        stack<string> operand_stack;

        vector<string>::iterator iter;

        for(iter = tokens.begin(); iter != tokens.end(); ++iter)

        {

            if(*iter == "+")//这里就不提取重复代码了，直接简单点 

            {

                string op1 = operand_stack.top();

                operand_stack.pop();

                string op2 = operand_stack.top();

                operand_stack.pop();

                //int ret = atoi(op2.c_str()) + atoi(op1.c_str());

                int ret = stoi(op2) + stoi(op1);//stoi为C++11才有 

                string result = to_string(ret);

                operand_stack.push(result);

                

            }

            else if(*iter == "-")

            {

                string op1 = operand_stack.top();

                operand_stack.pop();

                string op2 = operand_stack.top();

                operand_stack.pop();

                int ret = atoi(op2.c_str()) - atoi(op1.c_str());

                string result = to_string(ret);

                operand_stack.push(result);

                

            }

            else if(*iter == "*")

            {

                string op1 = operand_stack.top();

                operand_stack.pop();

                string op2 = operand_stack.top();

                operand_stack.pop();

                int ret = atoi(op2.c_str()) * atoi(op1.c_str());

                string result = to_string(ret);

                operand_stack.push(result);

                

            }

            else if(*iter == "/")

            {

                string op1 = operand_stack.top();

                operand_stack.pop();

                string op2 = operand_stack.top();

                operand_stack.pop();

                if( atoi(op1.c_str()) == 0)

                    return 0x7FFFFFF;

                int ret = atoi(op2.c_str()) / atoi(op1.c_str());

                string result = to_string(ret);

                operand_stack.push(result);

                

            }

            else

                operand_stack.push(*iter);

        }

        return atoi(operand_stack.top().c_str());

    

    }

 

};



int main(void)

{

    string strs[] = {"4", "13", "5", "/", "+"};

    vector<string> tokens(strs, strs+5);

    Solution solution;

    int ret = solution.evalRPN(tokens);

    cout<<ret<<endl;

    return 0;

}