The stack is an abstract data type that follows an order LIFO (last in first out) to evaluate any expression.
The element that is inserted at the last into the stack will be the first to get out of the stack.
Application of the stack:
1) Converting infix to postfix/prefix expression
2) parenthesis matching
3) Expression evaluation etc.
Explanation:
Only one stack is enough to evaluate any expression or to convert one form to another form of expression.
Suppose we have a postfix expression: 15 3 * 10 – 5 /
For evaluating this:
1) Push 15 in the stack, push 3 in the stack
2) when * operator comes, pop 15 and 3 from the stack
3) Push 15 * 3 = 45 in the stack
4) push 10 in the stack
5) when – operator occurs, pop 45 and 10 from the stack
6) push 45 – 10 = 35 in the stack
7) push 5 in the stack
8) when / operator comes, pop 35 and 5 from the stack
9) Push 35 / 5 = 7 in the stack
Concept:
Explanation:
A recursive problem like the Tower of Hanoi can be rewritten using system stack or userdefined stack
Recurrence relation of tower of Hanoi: T(n) = 2T(n  1) + 1
Additional Information
Number of moves required for n disc in a Tower of Hanoi is 2n – 1 = 27 – 1 = 127.
Stack underflow happens when one tries to pop (remove) an item from the stack when nothing is actually there to remove.
A stack can be implemented using two queues. Let stack to be implemented be ‘x’ and queues used to implement be ‘a’ and ‘b’.
Method 1 (By push operation)
This method makes sure that the newly entered element is always at the front of ‘a’, so that pop operation just dequeues from ‘a’. ‘b’ is used to put every new element at front of ‘b’.
Method 2 (By making pop operation costly)
In a push operation, the new element is always enqueued to a. In pop() operation, if b is empty then all the elements except the last, are moved to b. Finally, the last element is dequeued from a and returned.
Therefore Option 2 is correct
The Preorder traversal of a tree given below is:
The correct solution is 'option 1'.
Key Points
Algo Preorder(tree root)
{
}
Thus, the correct answer is: A B D F E C G I H J K L
Additional Information
Tree traversal  
Method flow 
Inorder  preorder  postorder 
Converse Inorder 
Converse Preorder  Converse Postorder 






A queue is also called a _____ system.
I. FIFO
II. LIFO
III. FILO
IV. LILO
Answer: Option 2
Explanation:
 For implementing recursive calls, Stack (LIFO) is required. Because for each call to the function, the caller function's temporary variables need to be pushed into the stack so that at the time of backtracking variables are easily accessible.
for example:
Consider the following Fibonacci example Where fib5 make a call to fib4 then fib5 will be pushed into the stack and then fib4 make a call to fib3, fib4 will be pushed into the stack and so on. and when fib1 and fib0 complete then backtracks to fib3 reusing the variable of fib3 which are stored in the stack.
Additional Information
Hash Table:
A hash table is a data structure that is used to store keys/value pairs. It uses a hash function to compute an index into an array in which an element will be inserted or searched. By using a good hash function, hashing can work well. Under reasonable assumptions, the average time required to search for an element in a hash table is O(1).
Queue:
The queue is FIFO (First in First out) data structure i.e. the first element that is added to the queue is the first one to be removed. Elements are always added to the back (Rear) and removed from the front.
Binary Tree:
A Binary tree consists of nodes which are having data part and a pointer pointing to the left subtree and a right pointer pointing to the right subtree.
If a tree is empty, it is represented by a null pointer.
Consider the following recursive function.
Int function (int x, int y) {
If (y <= 0) return x;
return function (y, x%y);
}
The above recursive function computes ______.
Consider x = 10 and y = 25
CASE1:
If (y <= 0) return x; // 25<=0 (false)
return function (y, x%y); return function(25, 10)
CASE 2:
x = 25, y= 10
If (y <= 0) return x; // 10<=0 (False)
return function (y, x%y); // return function (10, 5)
CASE 3:
x =10, y =5
If (y <= 0) return x; // 5<=0 (false)
return function (y, x%y); //return function(5, 0)
CASE 4:
x = 5, y =0
If (y <= 0) return x; // condition true, return 5
return function (y, x%y);
Output = 5
Which is greatest common divisor of 10 and 25.
So, given program computes GCD of x and y
191 Points
54 Points
53 Points
52 Points
49 Points