Facebook Software Development Engineer Interview Questions and Answers

Regex for email validation

• Start with a string of characters followed by @ symbol

• Followed by a string of characters and a period

• End with a string of characters with a length of 2-6 characters

• Allow for optional subdomains separated by periods

• Disallow special characters except for . and _ in username

Print prime numbers in a given range and optimize the solution.

• Use Sieve of Eratosthenes algorithm to generate prime numbers efficiently

• Start with a boolean array of size n+1, mark all as true

• Loop through the array and mark all multiples of each prime as false

• Print all the indexes that are still marked as true

Find angle between hour and minute hand in a clock given the time.

• Calculate the angle made by the hour hand with respect to 12 o'clock position

• Calculate the angle made by the minute hand with respect to 12 o'clock position

• Find the difference between the two angles and take the absolute value

• If the angle is greater than 180 degrees, subtract it from 360 degrees to get the smaller angle

To un-hash a string, use a reverse algorithm to convert the hash back to the original string.

• Create a reverse algorithm that takes the hash as input and outputs the original string

• Use the same logic as the hash function but in reverse order

• If the hash function used a specific algorithm, use the inverse of that algorithm to un-hash the string

Search an element in sorted rotated array.

• Use binary search to find the pivot point where the array is rotated.

• Divide the array into two subarrays and perform binary search on the appropriate subarray.

• Handle the cases where the target element is at the pivot point or not present in the array.

Find the Kth largest element in an array.

• Sort the array in descending order and return the element at index K-1.

• Use a max heap to find the Kth largest element efficiently.

• Implement a quickselect algorithm to find the Kth largest element in linear time.

Reverse a linked list in groups of n elements.

• Divide the linked list into groups of n elements.

• Reverse each group individually.

• Connect the reversed groups to form the final linked list.

• Handle cases where the number of elements is not a multiple of n.

• Example: 1->2->3->4->5->6->7->8, n=3 -> 3->2->1->6->5->4->8->7

Find the median of two sorted arrays.

• Merge the two arrays into one sorted array and find the median.

• Use binary search to find the median in O(log(min(m, n))) time complexity.

• Handle edge cases like empty arrays or arrays of different lengths.

Count number of paths from (0, 0) to (m, n) in a matrix while crossing through some indexes were not allowed.

• Use dynamic programming to solve the problem

• Create a 2D array to store the number of paths

• Traverse the matrix and update the array based on the allowed paths

• Return the value at the last index of the array

Print the level order traversal of binary tree in spiral form

• Perform level order traversal of the binary tree

• Alternate the direction of traversal for each level

• Use a stack to reverse the order of nodes in each level

• Print the nodes in the order of traversal

Find the maximum element in each subarray of size k in a given array.

• Iterate through the array from index 0 to n-k.

• For each subarray of size k, find the maximum element.

• Store the maximum elements in a separate array.

• Return the array of maximum elements.

To find the Kth largest element in two sorted arrays, we can use the merge step of merge sort algorithm.

• Merge the two arrays into a single sorted array using a modified merge sort algorithm.

• Return the Kth element from the merged array.

Merge two sorted arrays into one sorted array with expected time complexity of (m+n).

• Use a two-pointer approach to compare elements from both arrays and merge them into the first array.

• Start comparing elements from the end of both arrays and place the larger element at the end of the first array.

• Continue this process until all elements from the second array are merged into the first array.