# Data Structure Questions and Answers updated daily – Computer Knowledge

Data Structure Questions: Solved 242 Data Structure Questions and answers section with explanation for various online exam preparation, various interviews, Computer Knowledge Category online test. Category Questions section with detailed description, explanation will help you to master the topic.

## Data Structure Questions

221. Consider the following C function in which size is the number of elements in the array E:
The value returned by the function MyX is the

int MyX(int *E, unsigned int size)
{
int Y = 0;
int Z;
int i, j, k;

for (i = 0; i < size; i++)
Y = Y + E[i];

for (i = 0; i < size; i++)
for (j = i; j < size; j++)
{
Z = 0;
for (k = i; k <= j; k++)
Z = Z + E[k];
if (Z > Y)
Y = Z;
}
return Y;
}

Correct Ans:maximum possible sum of elements in any sub-array of array E.
Explanation:
The function does following
Y is used to store maximum sum seen so far and Z is used to store current sum
1) Initialize Y as sum of all elements
2) For every element, calculate sum of all subarrays starting with arr[i]. Store the current sum in Z. If Z is greater than Y, then update Y.
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222. Let P be a QuickSort Program to sort numbers in ascending order using the first element as pivot. Let t1 and t2 be the number of comparisons made by P for the inputs {1, 2, 3, 4, 5} and {4, 1, 5, 3, 2} respectively. Which one of the following holds?

Correct Ans:t1 > t2
Explanation:
When first element or last element is chosen as pivot, Quicksort's worst case occurs for the sorted arrays.

In every step of quick sort, numbers are divided as per the following recurrence.
T(n) = T(n-1) + O(n)
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223. Consider the directed graph given below. Which one of the following is TRUE?

Correct Ans:Both PSRQ and SPRQ are topological ordering
Explanation:
The graph doesn't contain any cycle, so there exist topological ordering.

P and S must appear before R and Q because there are edges from P to R and Q, and from S to R and Q.
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224. Consider a rooted Binary tree represented using pointers. The best upper bound on the time required to determine the number of subtrees having having exactly 4 nodes O(n^a Logn^b). Then the value of a + 10b is ________

Correct Ans:1
Explanation:
We can find the subtree with 4 nodes in O(n) time. Following can be a simple approach.

1) Traverse the tree in bottom up manner and find size of subtree rooted with current node
2) If size becomes 4, then print the current node.

int print4Subtree(struct Node *root)
{
if (root == NULL)
return 0;
int l = print4Subtree(root->left);
int r = print4Subtree(root->right);
if ((l + r + 1) == 4)
printf("%d ", root->data);
return (l + r + 1);
}
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225. Let G be a graph with n vertices and m edges. What is the tightest upper bound on the running time on Depth First Search of G? Assume that the graph is represented using adjacency matrix.

Correct Ans:O(n^2)
Explanation:
Depth First Search of a graph takes O(m+n) time when the graph is represented using adjacency list.

In adjacency matrix representation, graph is represented as an 'n x n' matrix. To do DFS, for every vertex, we traverse the row corresponding to that vertex to find all adjacent vertices (In adjacency list representation we traverse only the adjacent vertices of the vertex). Therefore time complexity becomes O(n^2)
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226. Suppose implementation supports an instruction REVERSE, which reverses the order of elements on the stack, in addition to the PUSH and POP instructions. Which one of the following statements is TRUE with respect to this modified stack?

Correct Ans:A queue can be implemented where ENQUEUE takes a sequence of three instructions and DEQUEUE takes a single instruction
Explanation:
To DEQUEUE an item, simply POP.
To ENQUEUE an item, we can do following 3 operations
1) REVERSE
2) PUSH
3) REVERSE
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227. Which one of the following correctly determines the solution of the recurrence relation with T(1) = 1?

T(n) = 2T(n/2) + Logn

Correct Ans:O(n)
Explanation:
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228. A priority queue is implemented as a Max-Heap. Initially, it has 5 elements. The level-order traversal of the heap is: 10, 8, 5, 3, 2. Two new elements 1 and 7 are inserted into the heap in that order. The level-order traversal of the heap after the insertion of the elements is:

Correct Ans:10, 8, 7, 3, 2, 1, 5
Explanation:
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229. Consider a hash table with 9 slots. The hash function is h(k) = k mod 9. The collisions are resolved by chaining. The following 9 keys are inserted in the order: 5, 28, 19, 15, 20, 33, 12, 17, 10. The maximum, minimum, and average chain lengths in the hash table, respectively, are

Correct Ans:3, 0, and 1
Explanation:
Following are values of hash function for all keys

5 --> 5
28 --> 1
19 --> 1 [Chained with 28]
15 --> 6
20 --> 2
33 --> 6 [Chained with 15]
12 --> 3
17 --> 8
10 --> 1 [Chained with 28 and 19]

The maximum chain length is 3. The keys 28, 19 and 10 go to same slot 1, and form a chain of length 3.

The minimum chain length 0, there are empty slots (0, 4 and 7).
Average chain length is (0 + 3 + 1 + 1 + 0 + 1 + 2 + 0 + 1)/9 = 1
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230. Consider the following pseudo code. What is the total number of multiplications to be performed?

D = 2
for i = 1 to n do
for j = i to n do
for k = j + 1 to n do
D = D * 3

Correct Ans:One-sixth of the product of the 3 consecutive integers.
Explanation:
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231. Consider the tree arcs of a BFS traversal from a source node W in an unweighted, connected, undirected graph. The tree T formed by the tree arcs is a data structure for computing.

Correct Ans:the shortest path from W to every vertex in the graph.
Explanation:
BFS always produces shortest paths from source to all other vertices in an unweighted graph. The reason is simple, in BFS, we first explore all vertices which are 1 edge away from source, then we explore all vertices which are 2 edges away from the source and so on. This property of BFS makes it useful in many algorithms like Edmonds–Karp algorithm.
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232. Suppose P, Q, R, S, T are sorted sequences having lengths 20, 24, 30, 35, 50 respectively. They are to be merged into a single sequence by merging together two sequences at a time. The number of comparisons that will be needed in the worst case by the optimal algorithm for doing this is ____.

Correct Ans:358
Explanation:
To merge two lists of size m and n, we need to do m+n-1 comparisons in worst case. Since we need to merge 2 at a time, the optimal strategy would be to take smallest size lists first. The reason for picking smallest two items is to carry minimum items for repetition in merging.

We first merge 20 and 24 and get a list of 44 using 43 worst case comparisons. Then we merge 30 and 35 into a list of 65 using 64 worst case comparisons. Then we merge 50 and 44 into a list of 94 using 93 comparisons. Finally we merge 94 and 65 using 158 comparisons. So total number of comparisons is 43 + 64 + 93 + 158 which is 358.
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233. Consider the pseudocode given below. The function DoSomething() takes as argument a pointer to the root of an arbitrary tree represented by the leftMostChild-rightSibling representation. Each node of the tree is of type treeNode.

typedef struct treeNode* treeptr;
struct treeNode {
treeptr leftMostChild, rightSibling;
};
int DoSomething (treeptr tree) {
int value = 0;
if (tree != NULL) {
if (tree->leftMostChild == NULL)
value = 1;
else
value = DoSomething(tree->leftMostChild);
value = value + DoSomething(tree->rightSibling);
}
return(value);
}

When the pointer to the root of a tree is passed as the argument to DoSomething, the value returned by the function corresponds to the

Correct Ans:number of leaf nodes in the tree.
Explanation:
The important thing to note in this question is tree’s representation. Tree is represented as leftmost child and right sibling form. So if the leftmost child is NULL for a node, then there is no child of this node. If we take a look at the function, we can notice that the function increments the “value” by 1 only for a leaf node.
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234. You have an array of n elements. Suppose you implement quicksort by always choosing the central element of the array as the pivot. Then the tightest upper bound for the worst case performance is

Correct Ans:O(n^2)
Explanation:
The middle element may always be an extreme element (minimum or maximum) in sorted order, therefore time complexity in worst case becomes O(n^2)
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235. The minimum number of arithmetic operations required to evaluate the polynomial P(X) = X^5 + 4X^3 + 6X + 5 for a given value of X using only one temporary variable.

Correct Ans:7
Explanation:
We can parenthesize the polynomial to minimize the number of operations.

Horner’s Method:

We get X(X2(X2 + 4) + 6) + 5 after parenthesization.

Following is sequence of operations to be used.
Note that we are allowed to use only one variable.
res = X*X
res = res + 4
res = X*res
res = X*res
res = res + 6
res = X*res
res = res + 5
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236. Let A be a square matrix of size n x n. Consider the following program. What is the expected output?

C = 100
for i = 1 to n do
for j = 1 to n do
{
Temp = A[i][j] + C
A[i][j] = A[j][i]
A[j][i] = Temp - C
}
for i = 1 to n do
for j = 1 to n do
Output(A[i][j]);

Correct Ans:The matrix A itself
Explanation:
If we take look at the inner statements of first loops, we can notice that the statements swap A[i][j] and A[j][i] for all i and j.

Since the loop runs for all elements, every element A[l][m] would be swapped twice, once for i = l and j = m and then for i = m and j = l. Swapping twice means the matrix does not change.
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237. Consider the following rooted tree with the vertex P labeled as root

The order in which the nodes are visited during in-order traversal is

Correct Ans:SQPTRWUV
Explanation:
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238. Consider a weighted complete graph G on the vertex set {v1, v2, ..vn} such that the weight of the edge (vi, vj) is 2|i-j|. The weight of a minimum spanning tree of G is

Correct Ans:2n - 2
Explanation:
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239. An element in an array X is called a leader if it is greater than all elements to the right of it in X. The best algorithm to find all leaders in an array

Correct Ans:Solves it in linear time using a right to left pass of the array
Explanation:
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240. Which one of the following in place sorting algorithms needs the minimum number of swaps?

Correct Ans:Selection sort
Explanation:
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