Check if it is possible to convert one string into another with given constraints

class Solution {

  public:

    int isItPossible(string S, string T, int M, int N) {

        // code here

        stack<char> st;

        for(int i=0;i<M || i<N ;i++)

        {

            if(T[i]=='A')

            {

                if(!st.empty() && st.top()!='A')

                    return 0;

                st.push(T[i]);

            }

            if(S[i]=='B')

            {

                if(!st.empty() && st.top()!='B')

                    return 0;

                st.push(S[i]);

            }

            if(!st.empty())

            {

                if((st.top() == 'A' && S[i] == 'A') || (st.top() == 'B' && T[i] == 'B'))

                st.pop();

            }

            else{

                //stack is empty

                if(S[i] == 'A' || T[i] == 'B')

                return 0;

            }

        }

        if(st.empty())

            return 1;

        else

            return 0;

    }

};