[C++]求表达式的逆波兰式

Posted 2022-05-28 HiveDark

// ReversePolish2.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <cstdlib>
#include <iostream>
#include <string>
#include <stack>
using std::cout;
using std::cin;
using std::endl;
using std::stack;
using std::string;

int priority(char c) 
    switch (c)
    
    case '+':
        return 1;
    case '-':
        return 1;
    case '*':
        return 2;
    case '/':
        return 2;
    default:
        return 0;
    


template<class T> string printStack(stack<T> stack) 
    string result = "";
    while (!stack.empty())
    
        result = stack.top()+result;
        stack.pop();
    
    return result;


string reversePolish(string inputStr) 
    stack<char> polish;
    stack<char> opStack;
    bool lastIsNum = false;
    for (int i = 0; i < inputStr.length(); i++) 
        char curChar = inputStr.at(i);
        //如果是数字
        if (curChar >= '0' && curChar <= '9') 
            if (!lastIsNum)
                polish.push(' ');
            polish.push(curChar);
            lastIsNum = true;
            continue;
        
        //否则为操作符
        else if (curChar == '(') 
            opStack.push(curChar);
        
        else if (!opStack.empty() && curChar == ')') 
            while (opStack.top() != '(')
            
                polish.push(opStack.top());
                opStack.pop();
            
            opStack.pop();
        
        else 
            if (!opStack.empty() && opStack.top() == '(') 
                opStack.push(curChar);
            
            else 
                if (!opStack.empty() && priority(curChar) > priority(opStack.top())) 
                    opStack.push(curChar);
                
                else 
                    while (!opStack.empty())
                    
                        if (opStack.top() == '(')
                            break;
                        if (priority(curChar) <= priority(opStack.top())) 
                            polish.push(opStack.top());
                            opStack.pop();
                        
                        else 
                            break;
                        
                    
                    opStack.push(curChar);
                
            
        
        lastIsNum = false;
    

    while (!opStack.empty()) 
        polish.push(opStack.top());
        opStack.pop();
    
    return printStack(polish);


int main()

    string str;
    cout << "请输入表达式: ";
    cin>>str;
    string result = reversePolish(str);
    cout << result<< endl;
    system("pause");
    return 0;

数字之间以空格隔开以便于下一步计算表达式的值

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