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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()
Solution
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 of the STACK 2 3 4 -> bottom of the STACK First 4 is inserted at the bottom. 4 Then 3 is inserted at the bottom. 4 3 Then 2 is inserted at the bottom. 4 3 2 Then 1 is inserted at the bottom 4 3 2 1 -> bottom of the STACK
So we need a function that inserts at the bottom of a stack using the above given basic stack function.
//Below is a recursive function that inserts an element at the bottom of a stack.
void insertAtBottom(struct sNode** top_ref, int item) { int temp; if(isEmpty(*top_ref)) { push(top_ref, item); } else { /* Hold all items in Function Call Stack until we reach end of the stack. When the stack becomes empty, the isEmpty(*top_ref) becomes true, the above if part is executed and the item is inserted at the bottom */ temp = pop(top_ref); insertAtBottom(top_ref, item); /* Once the item is inserted at the bottom, push all the items held in Function Call Stack */ push(top_ref, temp); } }
//Below is the function that reverses the given stack using insertAtBottom()
void reverse(struct sNode** top_ref) { int temp; if(!isEmpty(*top_ref)) { /* Hold all items in Function Call Stack until we reach end of the stack */ temp = pop(top_ref); reverse(top_ref); /* Insert all the items (held in Function Call Stack) one by one from the bottom to top. Every item is inserted at the bottom */ insertAtBottom(top_ref, temp); } }
//Below is a complete running program for testing above functions.