Calculate an expression using stack

Hi , i need to calculate an expression with +/- only , it will get result once it see '=' sign using stack linked list (for exp: 200-100=) ,result 100.

My code is not working , can someone enlighten me ? Thanks a lot :)



#include <iostream>

#include <cstdlib>

#include <cstring>

#include <iomanip>

#include <ctime>

using namespace std;


const int MAX =100;

int getResult(char *);


class Stack

{

    public:

        Stack();

        ~Stack();

        void push(int);

        int pop();

		int top ();

		

        bool isEmpty () const;

        


    private:


        struct Node;

        typedef Node* NodePtr ;


        struct Node

        {

            int data;

            NodePtr next;


        };

		

		NodePtr head;


};


Stack::Stack()

{

	head =NULL;	   

}

Stack::~Stack()

{

	//Let the compiler to do it

}

void Stack::push (int item)

{

	NodePtr pNew= new Node;

	pNew -> data = item;

	pNew -> next = head;

	head = pNew;

}


int Stack::pop()

{

	int item = NULL;

	try

	{

		if ( head == NULL)

			throw exception ();

		

		

		NodePtr temp = head;

		head = head ->next;

		

		item = temp ->data;

		

		delete temp;

	}

	catch (exception e)

	{

		cout << " Empty exception caught : Remove head failed "<< endl;

	}

	return item;

}

	


bool Stack::isEmpty () const

{

	return head == NULL;

}


int Stack::top ()

{

	if (!isEmpty())

		return head-> data;

}




int main ()

{

	char * exp = new char[MAX];

	cout <<"Enter an expression" <<endl;

   cin.getline(exp,MAX);

	int result = getResult (exp);

	

	cout <<result <<endl;


}



int getResult(char *exp)

{

	

	int i =0;

	int k,k1,k2;

	int result = 0;

	char *p;

	Stack s;

	p = &exp[0];

	

	// Calculate the result if last element is '=' sign

	while (*p!='=')

	{

		/* To remove spaces and tabs*/

		while (*p ==' ' || *p =='\t')

		{

			p++;

		}

		

		// If is operand

		if (isdigit(*p))

		{

			s.push(*p);

			

		}

		else

		{

		//If is operator +,-

				k1 = s.pop();

				k2 = s.pop();

				

			switch (*p)

			{

				case '+' : 	result =k1 + k2;

							

							

				case '-' :  result =k1 - k2;

							

				

				default : cout<<"\nInvalid operator. Program exit.";

						  

				

			// push back to stack

			s.push(result);

			}

					

		

		}

		

		p++; // Go to next element

		

	}

	

	

	result= s.pop();

	

	


	return result;

}