I have a C++ homework and I can't solve the password encryption function

Password Encryption
You are NOT allowed to use string operations Or array declaration in the encryption process. You are asked to declare lists stacks and Queues of type char from the STL to encrypt the password to get full score.
Encryption Algorithm
Step 1: (2 Points)
In this step the password characters will be inserted into list object to be ready for the coming encryption stages.
(objects to use → only list)
Step 2: (2 Points)
In this step, the first and last characters of the password (within (e.g H7l*lo → o7l*lH)
(objects to use → only list)
Step 3: (5 Points)
the list) will be reversed.
In this step, the ASCII value of the characters will be changed and the new characters (after changing the ASCII) will be in original order in the list.
The new ASCII value for each character will be calculated by adding to the current ASCII value the least significant digit from the ASCII of the next character.
(e.g. After step 2, the password is: oellH with ASCII values:
           o: 111,     7: 55,    l: 108,     *:42        l: 108,    H: 72
      The new ASCII values will be:
           t: 111+5,   ?: 55+8,  n: 108+2,   2:42+8      n: 108+2,  H: 72
      The password will be : t?n2nH
Note: The ASCII for the last character will not be changed because there is no character next to the last one.
(objects to use → list and stack OR list and queue OR both) give explanation for your choice.
Step 4: (4 Points)
Finally, in this step, the characters in even positions will be reversed and you keep the characters in the odd positions in their original order.
(e.g. After step 3, the password is : t?n2nH so after applying step 4 the password will be : n?n2tH (objects to use → list and stack OR list and queue OR both) give explanation for your choice

string UserAccountType::encryptPassword(string ep){
list<char> l;
stack<char> s;
for(int i=0;i<ep.size();i++)
l.push_back(ep[i]);

 char temp=l.front();
 l.pop_front();
 l.push_front(l.back());
 l.pop_back();
 l.push_back(temp);
 
while(!l.empty()){
s.push(l.back());
l.pop_back();
}

queue <char> q;
char x;
temp=s.top();
while(s.size() != 1){
x=s.top();
s.pop();
s.top()+=x%10;
q.push(x);
}
q.push(temp);

}