题解 | #计算表达式#

和简单计算器一模一样直接搬过来用了
#include<iostream>
#include<cstdio>
#include<stack>
#include<map>
#include<string>
using namespace std;

map<char, int> priority =
{
	{'+',1},{'-',1},
	{'*',2},{'/',2}
};

float getnum(int &index, string str) {  //将字符串化为整数
	float res = 0;
	while (str[index] >= '0' && str[index] <= '9') {
		res = res * 10 + str[index++] - '0';
	}
	return res;

}

float res(float a, float b, char c) {
	if (c == '+')
		return a + b;
	else if (c == '-')
		return a - b;
	else if (c == '*')
		return a * b;
	else 
		return a / b;

}


int main() 
{
	string str;
	while (getline(cin , str)) 
	{
		stack<float> num;
		stack<char> oper;   //分别标识数字栈和符号栈

		for (int i = 0; i < str.size(); i++) 
		{
			if (str[i] >= '0' && str[i] <= '9') 
			{
				num.push(getnum(i, str));
			}
			if (str[i] == '+' || str[i] == '-'||str[i] == '*'||str[i] == '/')
			{
				if (!oper.empty())
				{	
					if(priority[oper.top()] >= priority[str[i]])
					{
						while(priority[oper.top()] >= priority[str[i]] ) {  //将优先级高的全部弹出
							char c = oper.top();  oper.pop();  //弹出符号
							float b = num.top();   num.pop();		//弹出数字a
							float a = num.top();   num.pop();		//弹出数字b
							num.push(res(a, b, c));
							if (oper.empty()) break;
						}
						oper.push(str[i]);
					}
					else oper.push(str[i]);
				}
				else oper.push(str[i]);
			}
		}
		while (!oper.empty()) 
		{ 
			char c = oper.top();  oper.pop();  //弹出符号
			float b = num.top();   num.pop();		//弹出数字a
			float a = num.top();   num.pop();		//弹出数字b
			num.push(res(a, b, c));
		}
		printf("%.0f\n", num.top());
	}
}