直接用栈模拟，“-”顺序别弄错，不然会WA...

#include<iostream>
#include<string>
#include<cmath>
#include<stack>
using namespace std;
int main() {int n;while (cin >> n && n) {stack<double> stk;string str;for (int i = 0; i < n; i++) {cin >> str;if (str == "x") {stk.push(2);} else if (str == "sin") {double temp = stk.top();stk.pop();stk.push(sin(temp));} else if (str == "cos") {double temp = stk.top();stk.pop();stk.push(cos(temp));} else if (str == "tan") {double temp = stk.top();stk.pop();stk.push(tan(temp));} else if (str == "+") {double temp1 = stk.top();stk.pop();double temp2 = stk.top();stk.pop();stk.push(temp1 + temp2);} else if (str == "-") {double temp1 = stk.top();stk.pop();double temp2 = stk.top();stk.pop();stk.push(temp2 - temp1);} else if (str == "*") {double temp1 = stk.top();stk.pop();double temp2 = stk.top();stk.pop();stk.push(temp1 * temp2);}}if (fabs(stk.top()) < 0.00000001) {cout << "Identity" << endl;} else {cout << "Not an identity" << endl;}}
}