计蒜客 数据结构 栈 stack_expression C++

//stack_expression
//只处理加法和乘法的用栈实现的表达式求值，表达式长度为n
//表达式输入的中间不能有空格
#include<iostream>
#include<string>
#include<cassert>
using namespace std;
template<class Type> class Stack {
private:
    Type *urls;
    int max_size, top_index;
public:
    Stack(int length_input) {
        urls = new Type[length_input];
        max_size = length_input;
        top_index = -1;
    }
    ~Stack() {
        delete[] urls;
    }
    bool push(const Type &element) {
        if (top_index >= max_size - 1) {
            return false;
        }
        top_index++;
        urls[top_index] = element;
        return true;
    }
    bool pop() {
        if (top_index < 0) {
            return false;
        }
        top_index--;
        return true;
    }
    Type top() {
        assert(top_index >= 0);
        return urls[top_index];
    }
    bool empty() {
        if (top_index < 0) {
            return true;
        } else {
            return false;
        }
    }
};
bool precede(char a, char b) {
    if (a == '*') {
        return true;
    } else {
        return false;
    }
}
int operate(char theta, int a, int b) {
    if (theta == '+') {
        return a + b;
    } else {
        return a * b;
    }
}
void calc(Stack<int> &numbers, Stack<char> &operators) {
    int a = numbers.top();
    numbers.pop();
    int b = numbers.top();
    numbers.pop();
    numbers.push(operate(operators.top(),a,b));
    operators.pop();
}
int main() {
    int n;
    //n表示输入表达式的长度
    cin>>n;
    //numbers栈存储数据，operators栈存储运算符
    Stack<int> numbers(n);
    Stack<char> operators(n);
    string buffer;
    cin>>buffer;
    int i = 0;
    while(i<n){
        //用[]表示其中的元素
        if(isdigit(buffer[i])){
            //把char类型的字符转化成int类型的数字，让字符减去‘0’即可。
            numbers.push(buffer[i] - '0');
            i++;
        }
        else{
            //如果操作符号栈operators为空，或者当前运算符的优先级别高于栈顶的运算符
            //运算符之间的比较
           if(operators.empty() || precede(buffer[i], operators.top())) {
               operators.push(buffer[i]);
               i++;
           }
            else{
                calc(numbers,operators);
            }
        }
    }
    //即使字符串扫描完成后，栈中还是可能会有没运算的操作符和数字
     while(!operators.empty()){
        calc(numbers,operators);
     }
    cout<<numbers.top()<<endl;
    return 0;
}