Following questions have been asked in GATE CS 2007 exam.
1. The height of a binary tree is the maximum number of edges in any root to leaf path. The maximum number of nodes in a binary tree of height h is:
(A) 2^h -1
(B) 2^(h-1) - 1
(C) 2^(h+1) -1
(D) 2*(h+1)
Answer (C)
Maximum number of nodes will be there for a complete tree.
Number of nodes in a complete tree of height h = 1 + 2 + 2^2 + 2*3 + …. 2^h = 2^(h+1) – 1
2: The maximum number of binary trees that can be formed with three unlabelled nodes is:
(A) 1
(B) 5
(C) 4
(D) 3
Answer (B)
O
/ \
O O
(i)
O
/
O
/
O
(ii)
O
/
O
\
O
(iii)
O
\
O
\
O
(iv)
O
\
O
/
O
(v)
Note that nodes are unlabelled. If the nodes are labeled, we get more number of trees.
3. Which of the following sorting algorithms has the lowest worst-case complexity?
(A) Merge sort
(B) Bubble sort
(C) Quick sort
(D) Selection sort
Answer (A)
Worst case complexities for the above sorting algorithms are as follows:
Merge Sort -- nLogn
Bubble Sort -- n^2
Quick Sort -- n^2
Selection Sort -- n^2
4. The following postfix expression with single digit operands is evaluated using a stack:
8 2 3 ^ / 2 3 * + 5 1 * -
Note that ^ is the exponentiation operator. The top two elements of the stack after the first * is evaluated are:
(A) 6, 1
(B) 5, 7
(C) 3, 2
(D) 1, 5
Answer (A)
The algorithm for evaluating any postfix expression is fairly straightforward:
1. While there are input tokens left
o Read the next token from input.
o If the token is a value
+ Push it onto the stack.
o Otherwise, the token is an operator
(operator here includes both operators, and functions).
* It is known a priori that the operator takes n arguments.
* If there are fewer than n values on the stack
(Error) The user has not input sufficient values in the expression.
* Else, Pop the top n values from the stack.
* Evaluate the operator, with the values as arguments.
* Push the returned results, if any, back onto the stack.
2. If there is only one value in the stack
o That value is the result of the calculation.
3. If there are more values in the stack
o (Error) The user input has too many values.
Source for algorithm: https://en.wikipedia.org/wiki/Reverse_Polish_notation#The_postfix_algorithm
Let us run the above algorithm for the given expression.
First three tokens are values, so they are simply pushed. After pushing 8, 2 and 3, the stack is as follows
8, 2, 3
When ^ is read, top two are popped and power(2^3) is calculated
8, 8
When / is read, top two are popped and division(8/8) is performed
1
Next two tokens are values, so they are simply pushed. After pushing 2 and 3, the stack is as follows
1, 2, 3
When * comes, top two are popped and multiplication is performed.
1, 6
5. The inorder and preorder traversal of a binary tree are d b e a f c g and a b d e c f g, respectively. The postorder traversal of the binary tree is:
(A) d e b f g c a
(B) e d b g f c a
(C) e d b f g c a
(D) d e f g b c a
Answer (A)
Below is the given tree.
a
/ \
/ \
b c
/ \ / \
/ \ / \
d e f g
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