中缀表达式转后缀表达式以及计算后缀表达式的值(C++)

#include <vector>
#include <stack>
#include <iostream>
#include <algorithm>
#include <sstream>
using namespace std;

#define ERROR 0x3f3f

string cto_string(char c) {
	stringstream stream;
	stream << c;
	return stream.str();
}

int cmp(char a, char b) {
	if (a=='+') {
		if (b == '-' || b == '+') return 0;
		else if (b == '*' || b == '/') return -1;
		else if (b == '(') return 1; 
	} else if (a == '-') {
		if (b == '-' || b == '+') return 0;
		else if (b == '*' || b == '/') return -1;
		else if (b == '(') return 1; 
	} else if (a == '*') {
		if (b == '*' || b == '/') return 0;
		else if(b == '+' || b == '-' || b == '(') return 1;
	} else if (a == '/') {
		if (b == '*' || b == '/') return 0;
		else if(b == '+' || b == '-' || b == '(') return 1;
	} else {
		cout << "invalid sign" << endl; 
		return ERROR;
	}
} 


vector<string> res;
vector<string> InfixToSuffix(string infix) {
	stack<char> st;
	int i = 0;
	while (i < infix.length()) {
		if (infix[i] == '(') {
			st.push(infix[i]);
			i ++;
		} else if (infix[i] == ')') {
			while (!st.empty()) {
				char op = st.top();st.pop();
				if (op == '(') break;
				else res.push_back(cto_string(op));
				i ++;
			}
		} else if (isdigit(infix[i])) {
			int n = infix[i ++] - '0';
			while (i < infix.length() && isdigit(infix[i])) {
				n = n * 10 + (infix[i ++] - '0'); 
			}
			res.push_back(to_string(n));
		} else {
			if (st.empty()) st.push(infix[i]);
			else {
				if (cmp(infix[i], st.top()) > 0) st.push(infix[i]);
				else if (cmp(infix[i], st.top()) <= 0) {
					while (!st.empty()) {
						char op = st.top();st.pop();
						res.push_back(cto_string(op));
						if (cmp(infix[i], op) > 0) break;
					}
					st.push(infix[i]); 
				}
			}
			i ++;
		}
	}
	while (!st.empty()) {
		res.push_back(cto_string(st.top())); st.pop();
	}
	
	return res;
} 



//计算后缀表达式的值 
int calculate(vector<string>& a) {
    stack<int> st;
    for (auto x : a) {
        if (x == "+") {
            int a = st.top();st.pop();
            int b = st.top();st.pop();
            st.push(b + a);
        }
        else if (x == "-") {
            int a = st.top();st.pop();
            int b = st.top();st.pop();
        	st.push(b - a);
        }
        else if (x == "*") {
            int a = st.top();st.pop();
            int b = st.top();st.pop();
            st.push(b * a);
        }
        else if (x == "/") {
            int a = st.top();st.pop();
            int b = st.top();st.pop();
            st.push(b / a);
        } 
        else {
            st.push(stoi(x));
        }
            
	}
    return st.top();
}

void print(string a) {cout << a << " ";}
    
int main() {;
	string s;
	cin >> s;
	vector<string> t = InfixToSuffix(s);
	for_each(t.begin(), t.end(), print);cout << endl;
	int ans = calculate(t);
	cout << ans << endl;
	return 0;
}