Categories of Questions

## Array Questions Citadel

**Count of index pairs with equal elements in an array** Suppose, we have given an integer array. The problem “Count of index pairs with equal elements in an array” asks to find out the no of pair of indices (i,j) in such a way that arr[i]=arr[j] and i is not equal to j. Example arr[] = {2,3,1,2,3,1,4} 3 Explanation Pairs ...

**Longest subarray not having more than K distinct elements** The problem “Longest subarray not having more than K distinct elements” states that suppose you have an array of integers, the problem statement asks to find out the longest sub-array that having not greater than k different elements. Example arr[] = {4, 3, 5, 2, 1, 2, 0, 4, 5} ...

**Minimum operation to make all elements equal in array** The problem “Minimum operation to make all elements equal in array” states that you are given an array with some integers in it. You have to find out the minimum operations that can be done to make an array equal. Example [ 1,3,2,4,1] 3 Explanation Either 3 subtractions can be ...

**Difference between highest and least frequencies in an array** The problem “Difference between highest and least frequencies in an array” states that suppose that you have an integer array. The problem statement asks to find out the maximum difference between the highest frequency and lowest frequency of two distinct numbers in an array. Example arr[] = {1, 2, 3, ...

**k-th missing element in increasing sequence which is not present in a given sequence** The problem “k-th missing element in increasing sequence which is not present in a given sequence” states that you are given two arrays. One of them is arranged in ascending order and another normal unsorted array with number k. Find the kth missing element which is not present in normal ...

**Check if a given array contains duplicate elements within k distance from each other** The problem “Check if a given array contains duplicate elements within k distance from each other” states that we have to check for duplicates in given unordered array within the range of k. Here the value of k is smaller than the given array. Examples K = 3 arr[] = ...

**Find all pairs (a, b) in an array such that a % b = k** Problem Statement The problem “Find all pairs (a, b) in an array such that a % b = k” states that you are given an array of integers and an integer value called k. The problem statement asks to find out the pair in such a way that that x ...

**Queries on XOR of greatest odd divisor of the range** Problem Statement The problem “Queries on XOR of greatest odd divisor of the range” states that you are given an array of integer and query q, each query consists of a range. The problem statement asks to find out the XOR of the greatest odd divisor within the given range ...

**Three way partitioning of an array around a given range** Problem Statement You are given an array of integers and a range of lowValue and highValue. The problem “Three way partitioning of an array around a given range” asks to partition the array such that array will be divided into three parts. The partitions of the arrays will be: Elements ...

**Replace two consecutive equal values with one greater** Problem Statement Suppose you have an integer array. The problem “Replace two consecutive equal values with one greater” asks to replace all those pair values say ‘a’ which comes consecutively with a number “a+1” 1 greater than them (two consecutive numbers), such that even after the modification or repetition there ...

**Find a sorted subsequence of size 3 in linear time** Problem Statement The problem “Find a sorted subsequence of size 3 in linear time” states that you have an integer array. The problem statement asks to find out the three numbers in such a way that array[i] < array [k] < array[k], and i < j < k. Example arr[] ...

**Rearrange an array in order – smallest, largest, 2nd smallest, 2nd largest** Problem Statement Suppose you have an integer array. The problem “Rearrange an array in order – smallest, largest, 2nd smallest, 2nd largest, ..” asks to rearrange the array in such a way that the smallest number comes first and then the largest number, then second smallest and then the second ...

**Count pairs from two sorted arrays whose sum is equal to a given value x** Problem Statement “Count pairs from two sorted arrays whose sum is equal to a given value x” problem states that you are given two sorted arrays of integers and an integer value called sum. The problem statement asks to find out the total number of pair which sums up to ...

**Printing brackets in Matrix Chain Multiplication Problem** Problem Statement We need to find the order of multiplication of matrices such that the number of operations involved in the multiplication of all the matrices is minimized. Then we need to print this order i.e. printing brackets in matrix chain multiplication problem. Consider you have 3 matrices A, B, ...

**Insert Delete GetRandom** In Insert Delete GetRandom problem we need to design a data structure that supports all following operations in average O(1) time. insert(val): Inserts an item val to the set if not already present. remove(val): Removes an item val from the set if present. getRandom: Returns a random element from the current set ...

**Increasing Subsequence of Length three with Maximum Product** Problem Statement In the “Increasing Subsequence of Length three with Maximum Product” problem, we have given an array of positive integers. Find the subsequence of length 3 with the maximum product. Subsequence should be increasing. Input Format The first and only one line containing an integer N denoting the size ...

**Find the Maximum Repeating Number in Array** Problem Statement In the “Find the Maximum Repeating Number in Array” problem we have given an unsorted array of size N. Given array contains numbers in range {0, k} where k <= N. Find the number that is coming the maximum number of times in the array. Input Format The ...

**Count of Triplets With Sum Less than Given Value** Problem Statement We have given an array containing N number of elements. In the given array, Count the number of triplets with a sum less than the given value. Example Input a[] = {1, 2, 3, 4, 5, 6, 7, 8} Sum = 10 Output 7 Possible triplets are : ...

**Find Triplet in Array With a Given Sum** Problem Statement Given an array of integers, find the combination of three elements in the array whose sum is equal to a given value X. Here we will print the first combination that we get. If there is no such combination then print -1. Example Input N=5, X=15 arr[] = ...

**First Repeating Element** Problem Statement We have given an array that contains n integers. We have to find the first repeating element in the given array. If there is no repeated element then print “No repeating integer found”. Note: Repeating elements are those elements that come more than once. (Array may contain duplicates) ...

**A Product Array Puzzle** Problem Statement In a product array puzzle problem we need to construct an array where the ith element will be the product of all the elements in the given array except element at the ith position. Example Input 5 10 3 5 6 2 Output 180 600 360 300 900 ...

**Find the first Repeating Number in a Given Array** Problem Statement There can be multiple repeating numbers in an array but you have to find the first repeating number in a given array (occurring the second time). Example Input 12 5 4 2 8 9 7 12 5 6 12 4 7 Output 5 is the first repeating element ...

## String Questions Citadel

**Check if all Rows of a Matrix are Circular Rotations of Each Other** Problem Statement In the “Check if all Rows of a Matrix are Circular Rotations of Each Other” problem we have given a char matrix, write a program to find whether all rows are circular rotations of each other or not. If all rows are circular rotations of each other print ...

**Length of Longest valid Substring** Problem Statement In the “Length of Longest valid Substring” we have given a string that contains the opening and closing parenthesis only. Write a program that will find the longest valid parenthesis substring. Input Format The first and only one line containing a string s. Output Format The first and ...

## Tree Questions Citadel

**Construct Binary Tree from Given Inorder and Preorder Traversals** In this problem, we have inorder and preorder of the binary tree. We need to construct a binary tree from the given Inorder and Preorder traversals. Example Input: Inorder= [D, B, E, A, F, C] Preorder= [A, B, D, E, C, F] Output: Pre-order traversal of the tree formed by ...

**Validate Binary Search Tree** Problem In Validate Binary Search Tree problem we have given the root of a tree, we have to check if it is a binary search tree or not. Example : Output: true Explanation: The given tree is a binary search tree because all elements which are left to each subtree ...

## Queue Questions Citadel

**Priority Queue using doubly linked list** Problem Statement The problem “Priority Queue using doubly linked list” asks to implement the following functions of priority queue using doubly linked list. push(x, p) : Enqueue an element x with priority p in the priority queue at appropriate position. pop() : Remove and return the element with highest priority ...

## Matrix Questions Citadel

**Printing brackets in Matrix Chain Multiplication Problem** Problem Statement We need to find the order of multiplication of matrices such that the number of operations involved in the multiplication of all the matrices is minimized. Then we need to print this order i.e. printing brackets in matrix chain multiplication problem. Consider you have 3 matrices A, B, ...

**Check if all Rows of a Matrix are Circular Rotations of Each Other** Problem Statement In the “Check if all Rows of a Matrix are Circular Rotations of Each Other” problem we have given a char matrix, write a program to find whether all rows are circular rotations of each other or not. If all rows are circular rotations of each other print ...

## Other Questions Citadel

**Print n terms of Newman-Conway Sequence** Problem Statement The problem “Print n terms of Newman-Conway Sequence” states that you are given an integer “n”. Find the first n terms of Newman-Conway Sequence then print them. Example n = 6 1 1 2 2 3 4 Explanation All the terms which are printed follow the Newman-Conway Sequence ...

**Sliding Window Maximum** In Sliding Window Maximum problem we have given an array nums, for each contiguous window of size k, find the maximum element in the window. Example Input nums[] = {1,3,-1,-3,5,3,6,7} k = 3 Output {3,3,5,5,6,7} Explanation Naive Approach for Sliding Window Maximum For every contiguous window of size k, traverse ...

**LRU Cache Implementation** Least Recently Used (LRU) Cache is a type of method which is used to maintain the data such that the time required to use the data is the minimum possible. LRU algorithm used when the cache is full. We remove the least recently used data from the cache memory of ...

**Serialize and Deserialize Binary Tree** We have given a binary tree containing N number of nodes where each node has some value. We need to serialize and deserialize the binary tree. Serialize The process of storing a tree in a file without disturbing its structure is called serialization. DeserializeSerialize and Deserialize Binary Tree The process ...