题解 | 计算表达式

#include <stdio.h>
#include <string>
#include <stack>
#include <map>
using namespace std;

int main(){
	char str[1024];
	scanf("%s",str);
	map<char,int> opPriority = {
		{'\0',0},
		{'+',1},{'-',1},
		{'*',2},{'/',2},
	};
	stack<char> opStack;
	stack<float> numStack;
	string st_num;
	for(int i=0;;i++){	//拼凑整数 
		if(str[i]>='0'&&str[i]<='9'){
			st_num.push_back(str[i]);
		}else{	//操作符或结束符 ,也标志上一个数字拼凑完成 
			float num = stof(st_num);
			numStack.push(num);
			st_num = "";
			while(!opStack.empty()&&opPriority[str[i]]<=opPriority[opStack.top()]){	//如果当前字符优先级小于等于栈顶 ,循环进行操作 
				float r = numStack.top(); numStack.pop();
				float l = numStack.top(); numStack.pop();
				char op = opStack.top(); opStack.pop();
				if(op=='+'){
					numStack.push(l+r);
				}else if(op=='-'){
					numStack.push(l-r);
				}else if(op=='*'){
					numStack.push(l*r);
				}else if(op=='/'){
					numStack.push(l/r);
				}
			}
			if(str[i]=='\0'){
				printf("%d",(int)numStack.top());
				break;
			} else{
				opStack.push(str[i]);
			}
		}
	}
	return 0;
}