Question 1
What is the worst case time complexity for search, insert and delete operations in a general Binary Search Tree?
A
O(n) for all
B
O(Logn) for all
C
O(Logn) for search and insert, and O(n) for delete
D
O(Logn) for search, and O(n) for insert and delete
Binary Search Trees    
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Question 1 Explanation: 
In skewed Binary Search Tree (BST), all three operations can take O(n). See the following example BST and operations.
          10
        /
       20
      /
     30
    / 
   40

Search 40. 
Delete 40
Insert 50.
Question 2
In delete operation of BST, we need inorder successor (or predecessor) of a node when the node to be deleted has both left and right child as non-empty. Which of the following is true about inorder successor needed in delete operation?
A
Inorder Successor is always a leaf node
B
Inorder successor is always either a leaf node or a node with empty left child
C
Inorder successor may be an ancestor of the node
D
Inorder successor is always either a leaf node or a node with empty right child
Binary Search Trees    
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Question 2 Explanation: 
Let X be the node to be deleted in a tree with root as 'root'. There are three cases for deletion 1) X is a leaf node: We change left or right pointer of parent to NULL (depending upon whether X is left or right child of its parent) and we delete X 2) One child of X is empty: We copy values of non-empty child to X and delete the non-empty child 3) Both children of X are non-empty: In this case, we find inorder successor of X. Let the inorder successor be Y. We copy the contents of Y to X, and delete Y. Sp we need inorder successor only when both left and right child of X are not empty. In this case, the inorder successor Y can never be an ancestor of X. In this case, the inorder successor is the leftmost node in right subtree of X. Since it is leftmost node, the left child of Y must be empty.
Question 3
We are given a set of n distinct elements and an unlabeled binary tree with n nodes. In how many ways can we populate the tree with the given set so that it becomes a binary search tree? (GATE CS 2011)
A
0
B
1
C
n!
D
(1/(n+1)).2nCn
Binary Search Trees    
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Question 3 Explanation: 
There is only one way. The minimum value has to go to the leftmost node and the maximum value to the rightmost node. Recursively, we can define for other nodes.
Question 4
How many distinct binary search trees can be created out of 4 distinct keys?
A
4
B
14
C
24
D
42
Binary Search Trees    
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Question 4 Explanation: 
See question 2 of http://www.geeksforgeeks.org/data-structures-and-algorithms-set-23/ for explanation. The link also has a generalized solution.
Question 5
Which of the following traversal outputs the data in sorted order in a BST?
A
Preorder
B
Inorder
C
Postorder
D
Level order
Binary Search Trees    
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Question 5 Explanation: 
Inorder traversal of a BST outputs data in sorted order. Read here for details.
Question 6
Suppose the numbers 7, 5, 1, 8, 3, 6, 0, 9, 4, 2 are inserted in that order into an initially empty binary search tree. The binary search tree uses the usual ordering on natural numbers. What is the in-order traversal sequence of the resultant tree?
A
7 5 1 0 3 2 4 6 8 9
B
0 2 4 3 1 6 5 9 8 7
C
0 1 2 3 4 5 6 7 8 9
D
9 8 6 4 2 3 0 1 5 7
Binary Search Trees    
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Question 6 Explanation: 
In-order traversal of a BST gives elements in increasing order. So answer c is correct without any doubt.
Question 7
The following numbers are inserted into an empty binary search tree in the given order: 10, 1, 3, 5, 15, 12, 16. What is the height of the binary search tree (the height is the maximum distance of a leaf node from the root)? (GATE CS 2004)
A
2
B
3
C
4
D
6
Binary Search Trees    
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Question 7 Explanation: 
Constructed binary search tree will be..
                    10
                  /     \
                 1       15
                 \      /  \
                  3    12   16
                    \
                     5
Question 8
The preorder traversal sequence of a binary search tree is 30, 20, 10, 15, 25, 23, 39, 35, 42. Which one of the following is the postorder traversal sequence of the same tree?
A
10, 20, 15, 23, 25, 35, 42, 39, 30
B
15, 10, 25, 23, 20, 42, 35, 39, 30
C
15, 20, 10, 23, 25, 42, 35, 39, 30
D
15, 10, 23, 25, 20, 35, 42, 39, 30
Binary Search Trees    
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Question 8 Explanation: 
The following is the constructed tree
            30
         /      \
        20       39 
       /  \     /  \
     10    25  35  42  
      \   /
      15 23
Question 9
Consider the following Binary Search Tree

               10
             /    \
            5      20
           /      /  \           
          4     15    30
               /  
              11       
If we randomly search one of the keys present in above BST, what would be the expected number of comparisons?
A
2.75
B
2.25
C
2.57
D
3.25
Binary Search Trees    
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Question 9 Explanation: 
Expected number of comparisons = (1*1 + 2*2 + 3*3 + 4*1)/7 = 18/7 = 2.57
Question 10
Which of the following traversals is sufficient to construct BST from given traversals 1) Inorder 2) Preorder 3) Postorder
A
Any one of the given three traversals is sufficient
B
Either 2 or 3 is sufficient
C
2 and 3
D
1 and 3
Binary Search Trees    
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Question 10 Explanation: 
When we know either preorder or postorder traversal, we can construct the BST. Note that we can always sort the given traversal and get the inorder traversal. Inorder traversal of BST is always sorted.
Question 11
Consider the following code snippet in C. The function print() receives root of a Binary Search Tree (BST) and a positive integer k as arguments.
// A BST node
struct node {
    int data;
    struct node *left, *right;
};

int count = 0;

void print(struct node *root, int k)
{
    if (root != NULL && count <= k)
    {
        print(root->right, k);
        count++;
        if (count == k)
          printf("%d ", root->data);
       print(root->left, k);
    }
}
What is the output of print(root, 3) where root represent root of the following BST.
                   15
                /     \
              10      20
             / \     /  \
            8  12   16  25   
A
10
B
16
C
20
D
20 10
Binary Search Trees    
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Question 11 Explanation: 
The code mainly finds out k'th largest element in BST, see K’th Largest Element in BST for details.
Question 12
Consider the same code as given in above question. What does the function print() do in general? The function print() receives root of a Binary Search Tree (BST) and a positive integer k as arguments.
// A BST node
struct node {
    int data;
    struct node *left, *right;
};

int count = 0;

void print(struct node *root, int k)
{
    if (root != NULL && count <= k)
    {
        print(root->right, k);
        count++;
        if (count == k)
          printf("%d ", root->data);
       print(root->left, k);
    }
}
A
Prints the kth smallest element in BST
B
Prints the kth largest element in BST
C
Prints the leftmost node at level k from root
D
Prints the rightmost node at level k from root
Binary Search Trees    
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Question 12 Explanation: 
The function basically does reverse inorder traversal of the given Binary Search Tree. The reverse inorder traversal produces data in reverse sorted order. Whenever a nod is visited, count is incremented by 1 and data of a node is printed only when count becomes k.
Question 13
You are given the postorder traversal, P, of a binary search tree on the n elements 1, 2, ..., n. You have to determine the unique binary search tree that has P as its postorder traversal. What is the time complexity of the most efficient algorithm for doing this?
A
O(Logn)
B
O(n)
C
O(nLogn)
D
none of the above, as the tree cannot be uniquely determined.
Binary Search Trees    GATE CS 2008    
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Question 13 Explanation: 
One important thing to note is, it is Binary Search Tree, not Binary Tree. In BST, inorder traversal can always be obtained by sorting all keys. See method 2 of http://www.geeksforgeeks.org/construct-bst-from-given-preorder-traversa/ for details. Same technique can be used for postorder traversal.
Question 14
Suppose we have a balanced binary search tree T holding n numbers. We are given two numbers L and H and wish to sum up all the numbers in T that lie between L and H. Suppose there are m such numbers in T. If the tightest upper bound on the time to compute the sum is O(nalogb n + mc logd n), the value of a + 10b + 100c + 1000d is ____.
A
60
B
110
C
210
D
50
Binary Search Trees    GATE-CS-2014-(Set-3)    
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Question 14 Explanation: 
int getSum(node *root, int L, int H)
{
   // Base Case
   if (root == NULL)
      return 0;

   if (root->key < L)        
       return getSum(root->right, L, H);

  if (root->key > H)
      return getSum(root->left, L, H)

   if (root->key >= L && root->key <=H)        
      return getSum(root->left, L, H) + root->key +
             getSum(root->right, L, H);
}
The above always takes O(m + Logn) time. Note that the code first traverses across height to find the node which lies in range.  Once such a node is found, it recurs for left and right children. Two recursive calls are made only if the node is in range. So for every node that is in range, we make at most one extra call (here extra call means calling for a node that is not in range).
Question 15
Let T(n) be the number of different binary search trees on n distinct elements. Then GATECS2003Q7, where x is
A
n-k+1
B
n-k
C
n-k-1
D
n-k-2
Binary Search Trees    GATE-CS-2003    
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Question 15 Explanation: 
The idea is to make a key root, put (k-1) keys in one subtree and remaining n-k keys in other subtree. A Binary Search Tree (BST) is a tree in which all the nodes follow the below-mentioned properties −
  • The left sub-tree of a node has a key less than or equal to its parent node's key.
  • The right sub-tree of a node has a key greater than to its parent node's key.
Now construction binary search trees from n distinct number- Lets for simplicity consider n distinct numbers as first n natural numbers (starting from 1) If n=1 We have only one possibility, therefore only 1 BST. If n=2 We have 2 possibilities , when smaller number is root and bigger number is the right child or second when the bigger number is root and smaller number as left child.   parul_1 If n=3 We have 5 possibilities. Keeping each number first as root and then arranging the remaining 2 numbers as in case of n=2. parul_2   If n=4 We have 14 possibilities. Taking each number as root and arranging smaal numbers as left subtree and larger numbers as right subtree. parul_4 Thus we can conclude that with n distinct numbers, if we take ‘k’ as root then all the numbers smaller than k will left subtree and numbers larger than k will be right subtree where the the right subtree and left subtree will again be constructed recursively like the root. Therefore, parul5   This solution is contributed by Parul Sharma.
Question 16
What are the worst-case complexities of insertion and deletion of a key in a binary search tree?
A
Θ(logn) for both insertion and deletion
B
Θ(n) for both insertion and deletion
C
Θ(n) for insertion and Θ(logn) for deletion
D
Θ(logn) for insertion and Θ(n) for deletion
Binary Search Trees    GATE-CS-2015 (Set 1)    
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Question 16 Explanation: 
The time taken by search, insert and delete on a BST is always proportional to height of BST. Height may become O(n) in worst case.
Question 17
While inserting the elements 71, 65, 84, 69, 67, 83 in an empty binary search tree (BST) in the sequence shown, the element in the lowest level is
A
65
B
67
C
69
D
83
Binary Search Trees    GATE-CS-2015 (Set 3)    
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Question 17 Explanation: 
Here is The Insertion Algorithm For a Binary Search Tree :
Insert(Root,key)
{
    if(Root is NULL)
        Create a Node with value as key and return
    Else if(Root.key <= key)
        Insert(Root.left,key)
    Else
        Insert(Root.right,key)
}
Creating the BST one by one using the above algorithm in the below given image : pranjul_23 This solution is contributed by Pranjul Ahuja.
Question 18
The number of ways in which the numbers 1, 2, 3, 4, 5, 6, 7 can be inserted in an empty binary search tree, such that the resulting tree has height 6, is _____________ Note: The height of a tree with a single node is 0. [This question was originally a Fill-in-the-Blanks question]
A
2
B
4
C
64
D
32
Binary Search Trees    GATE-CS-2016 (Set 2)    
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Question 18 Explanation: 
To get height 6, we need to put either 1 or 7 at root. So count can be written as T(n) = 2*T(n-1) with T(1) = 1

    7
   / 
 [1..6]  

    1
      \
     [2..7] 
Therefore count is 26 = 64 Another Explanation: Consider these cases, 1 2 3 4 5 6 7 1 2 3 4 5 7 6 1 7 6 5 4 3 2 1 7 6 5 4 2 3 7 6 5 4 3 2 1 7 6 5 4 3 1 2 7 1 2 3 4 5 6 7 1 2 3 4 6 5 For height 6, we have 2 choices. Either we select the root as 1 or 7. Suppose we select 7. Now, we have 6 nodes and remaining height = 5. So, now we have 2 ways to select root for this sub-tree also. Now, we keep on repeating the same procedure till remaining height = 1 For this last case also, we have 2 ways. Therefore, total number of ways = 26= 64
Question 19
Suppose that we have numbers between 1 and 100 in a binary search tree and want to search for the number 55. Which of the following sequences CANNOT be the sequence of nodes examined?
A
{10, 75, 64, 43, 60, 57, 55}
B
{90, 12, 68, 34, 62, 45, 55}
C
{9, 85, 47, 68, 43, 57, 55}
D
{79, 14, 72, 56, 16, 53, 55}
Binary Search Trees    GATE IT 2006    
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Question 19 Explanation: 
In BST on right side of parent number should be greater than it, but in C after 47, 43 appears that is wrong.
Question 20
When searching for the key value 60 in a binary search tree, nodes containing the key values 10, 20, 40, 50, 70 80, 90 are traversed, not necessarily in the order given. How many different orders are possible in which these key values can occur on the search path from the root to the node containing the value 60?
A
35
B
64
C
128
D
5040
Binary Search Trees    Tree Traversals    Gate IT 2007    
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Question 20 Explanation: 
There are two set of values, smaller than 60 and greater than 60. Smaller values 10, 20, 40 and 50 are visited, means they are visited in order. Similarly, 90, 80 and 70 are visited in order.
= 7!/(4!3!)
= 35
Question 21
A Binary Search Tree (BST) stores values in the range 37 to 573. Consider the following sequence of keys.

I. 81, 537, 102, 439, 285, 376, 305
II. 52, 97, 121, 195, 242, 381, 472
III. 142, 248, 520, 386, 345, 270, 307
IV. 550, 149, 507, 395, 463, 402, 270

Suppose the BST has been unsuccessfully searched for key 273. Which all of the above sequences list nodes in the order in which we could have encountered them in the search?
A
II and III only
B
I and III only
C
III and IV only
D
III only
Binary Search Trees    Gate IT 2008    
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Question 21 Explanation: 
Key to be searched 273:
  • I) 81, 537, 102, 439, 285, 376, 305 is not correct We cannot go to 376 from 285 as 273 is smaller than 285.
  • II) 52, 97, 121, 195, 242, 381, 472 is not correct. We cannot go to 472 from 381 as 273 is smaller than 381.
  • III) 142, 248, 520, 386, 345, 270, 307 is correct 2008_71_sol
  • 550, 149, 507, 395, 463, 402, 270 is not correct. We cannot go to 463 from 395 in search of 273
Question 22
A Binary Search Tree (BST) stores values in the range 37 to 573. Consider the following sequence of keys.
I. 81, 537, 102, 439, 285, 376, 305
II. 52, 97, 121, 195, 242, 381, 472
III. 142, 248, 520, 386, 345, 270, 307
IV. 550, 149, 507, 395, 463, 402, 270
Which of the following statements is TRUE?
A
I, II and IV are inorder sequences of three different BSTs
B
I is a preorder sequence of some BST with 439 as the root
C
II is an inorder sequence of some BST where 121 is the root and 52 is a leaf
D
IV is a postorder sequence of some BST with 149 as the root
Binary Search Trees    Gate IT 2008    
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Question 22 Explanation: 
A: I and IV are not in ascending order
B: If 439 is root, It should be 1st element in preorder
D: IV is a postorder sequence of some BST with 149 as the root, => False
Question 23
How many distinct BSTs can be constructed with 3 distinct keys?
A
4
B
5
C
6
D
9
Binary Search Trees    Gate IT 2008    
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Question 23 Explanation: 
2nCn/(n+1) = 6C3/4 = 5
There are 23 questions to complete.

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