Stack Reversal

https://www.geeksforgeeks.org/reverse-a-stack-using-recursion/
Write a program to reverse a stack using recursion. You are not allowed to use loop constructs like while, for..etc, and you can only use the following ADT functions on Stack S:
isEmpty(S)
push(S)
pop(S)

The idea of the solution is to hold all values in Function Call Stack until the stack becomes empty. When the stack becomes empty, insert all held items one by one at the bottom of the stack.

For example, let the input stack be

    1  <-- top
    2
    3
    4   

First 4 is inserted at the bottom.
    4 <-- top

Then 3 is inserted at the bottom
    4 <-- top    
    3

Then 2 is inserted at the bottom
    4 <-- top    
    3 
    2
     
Then 1 is inserted at the bottom
    4 <-- top    
    3 
    2
    1

So we need a function that inserts at the bottom of a stack using the above given basic stack function.

void insertAtBottom((): First pops all stack items and stores the popped item in function call stack using recursion. And when stack becomes empty, pushes new item and all items stored in call stack.

void reverse(): This function mainly uses insertAtBottom() to pop all items one by one and insert the popped items at the bottom

   static void insert_at_bottom(char x)

    {

  

        if(st.isEmpty())

            st.push(x);

  

        else

        {

              

            // All items are held in Function

            // Call Stack until we reach end

            // of the stack. When the stack becomes

            // empty, the st.size() becomes 0, the

            // above if part is executed and 

            // the item is inserted at the bottom

            char a = st.peek();

            st.pop();

            insert_at_bottom(x);

  

            // push allthe items held 

            // in Function Call Stack

            // once the item is inserted 

            // at the bottom

            st.push(a);

        }

    }

      

    // Below is the function that 

    // reverses the given stack using

    // insert_at_bottom()

    static void reverse()

    {

        if(st.size() > 0)

        {

              

            // Hold all items in Function

            // Call Stack until we

            // reach end of the stack 

            char x = st.peek();

            st.pop();

            reverse();

              

            // Insert all the items held 

            // in Function Call Stack

            // one by one from the bottom

            // to top. Every item is

            // inserted at the bottom 

            insert_at_bottom(x);

        }

    }

http://codingrecipies.blogspot.com/2014/01/stack-reversal.html
   Using recursion start from the bottom most element in the stack using the 1st Function Call(reverse()) , push each element to the bottom of the stack through recursion using another function call(pushToBottom()).

public void reverseStack(Stack<Integer> stack){
  if(stack.isEmpty())
   return ;

  int val= stack.pop();
  reverseStack(stack);

  pushToBottom(stack,val);
  return ;
 }

 /**
  * Pushes the incoming element or item to the bottom of the stack 
  */
 private void pushToBottom(Stack<Integer> stack,int item){
  if(stack.isEmpty()){
   stack.push(item);
   return ;
  }

  int val= stack.pop();
  pushToBottom(stack,item);
  stack.push(val);
 }

https://www.geeksforgeeks.org/reverse-stack-without-using-extra-space/