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;
}