加减乘除实现模板：

``````#include<queue>
#include<stack>
#include<cstdio>
#include<map>
#include<iostream>
using namespace std;

struct node{
double num;
char oper;
bool if_num;
node(double num,char oper, bool if_num):num(num),oper(oper),if_num(if_num){}
};

queue<node> que;
stack<node> sta;
map<char,int> map_;
void init(){
while(!sta.empty()) sta.pop();
while(!que.empty()) que.pop();

map_['+'] = 0;
map_['-'] = 0;
map_['*'] = 1;
map_['/'] = 1;
}
//中缀转后缀
void change(string input){

string::iterator it = input.begin();
for(;it!=input.end();){
if(*it>= '0' && *it <='9'){
double num = *it - '0'; it++;
while(it!=input.end() && *it>= '0' && *it <='9'){
num = num*10 + (*it - '0');
it++;
}
node temp(num,' ',true);
que.push(temp);
}else{
node temp(0,*it,false);
//			node temp2 = sta.top();
//如果是有括号的话，那么遇到左括号‘（’则压入
// 遇到右括号‘)’则则弹出直到遇到第一个左括号
while(!sta.empty() && map_[sta.top().oper]>=map_[*it]){
que.push(sta.top());
sta.pop();
}
sta.push(temp);
it++;
}
}

while(!sta.empty()){
que.push(sta.top()); sta.pop();
}
}
//将后缀表达式调出来计算
double cal(){

while(!que.empty()){
node temp = que.front(); que.pop();
if(temp.if_num == true){
sta.push(temp);continue;
}else{
node a = sta.top();sta.pop();
node b = sta.top();sta.pop();
double ans = 0;
if(temp.oper == '+'){
ans = a.num + b.num;
} else if(temp.oper == '-'){
ans = b.num - a.num;
}else if(temp.oper == '*'){
ans = b.num * a.num;
}else if(temp.oper == '/'){
ans = b.num / a.num;
}
sta.push(node(ans,' ',true));
}
}
return sta.top().num;
}

int main(){

char str[100];
while(gets(str)!=NULL){
init();
string input(str);
string::iterator it = input.begin();
for(;it!=input.end();){
if(*it == ' '){
input.erase(it++);
}else it++;
}
cout<<input<<endl;
change(input);
double ans = cal();
printf("%.3lf\n",ans);
}
return 0;
}``````