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Initial commit — imported from GitHub
2026-08-25 19:14:07 -06:00
---
layout: page
title: EPI
---
Legend
======
- Solved
- Attempted
- Needs Revisit
Solutions
=========
Primitive Types
---------------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Computing the parity of a word | a | b | c | d
| Swap bits | | |
| Reverse bits | | |
| Find a closest integer with the same weight | | |
| Compute *x × y* without arithmetical operators | | |
| Compute *x/y* | | |
| Compute *x^y* | | |
| Reverse digits | | |
| Check if a decimal integer is a palindrome | | |
| Generate uniform random numbers | | |
| Rectangle intersection | | |
Arrays
------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| The Dutch national flag problem | | |
| Increment an arbitrary-precision integer | | |
| Multiply two arbitrary-precision integers | | |
| Advancing through an array | | |
| Delete a key from an array | | |
| Delete duplicates from a sorted array | | |
| Buy and sell a stock once | | |
| Buy and sell a stock twice | | |
| Enumerate all primes to *n* | | |
| Permute the elements of an array | | |
| Compute the next permutation | | |
| Sample offline data | | |
| Sample online data | | |
| Compute a random permutation | | |
| Compute a random subset | | |
| Generate nonuniform random numbers | | |
| The Sudoku checker problem | Solved | 3-4 hrs | n/a | apr 29 | Spent 1 hr just on row and col check, looked up box validation
| Compute the spiral ordering of a 2D array | Attempted | n/a | solve by simulation |
| Rotate a 2D array | Solved | 2hrs | transpose and flip horizontally | apr 29 | fav problems
| Compute rows in Pascal’s Triangle | | |
Strings
-------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Interconvert strings and integers | | |
| Base conversion | | |
| Compute the spreadsheet column encoding | | |
| Replace and remove | | |
| Solved palindromicity | | |
| Reverse all the words in a sentence | | |
| Compute all mnemonics for a phone number | | |
| The look-and-say problem | | |
| Convert from Roman to decimal | | |
| Compute all valid IP addresses | | |
| Write a string sinusoidally | | |
| Implement run-length encoding | | |
| Implement the UNIX `tail` command | | |
| Find the first occurrence of a substring | | |
Linked List
-----------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Merge two sorted lists | Solved | 1hr | iteratively, recursively | Apr 28 | deep dive on recursive solution
| Reverse a singly linked list | Solved | 30m | Iteratively | May1 | solve with recursion
| Reverse a single sublist | Attempted | n/a | n/a | May 2 | don't quite get the connection and tail variable
| Solved for cyclicity | | |
| Solved for overlapping lists—lists are cycle-free | | | | |
| Solved for overlapping lists—lists may have cycles | | | | |
| Delete a node from a singly linked list | | |
| Remove the *k*th last element from a list | | |
| Remove duplicates from a sorted list | | | | |
| Implement cyclic right shift for singly linked lists | | | | |
| Implement even-odd merge | | | | |
| Solved whether a singly linked list is palindromic | | | | |
| Implement list pivoting | | | | |
| Add list-based integers | | | | |
Stacks and Queues
-----------------
### Stacks
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Implement a stack with max API | | | | |
| Evaluate RPN expressions | | | | |
| Solved a string over “{,},(,),[,]” for well-formedness | | | | |
| Normalize pathnames | | | | |
| BST keys in sort order | | | | |
| Search a postings list | | | | |
| Compute buildings with a sunset view | | | | |
| Sort a stack | | | | |
### Queues
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Compute binary tree nodes in order of increasing depth | | | | |
| Implement a circular queue | | | | |
| Implement a queue using stacks | | | | |
| Implement a queue with max API | | | | |
Binary Trees
------------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Solved if a binary tree is balanced | Solved | 20min | Recursion | Apr30
| Solved if a binary tree is symmetric | Solved | 1hr | Recursion | May1 | If you need to compare 2 subtrees, use helper(left, right)
| Compute the lowest common ancestor in a binary tree | Solved | 45min | Recursion | Mar27, May1 |
| Compute the LCA when nodes have parent pointers | | | | |
| Sum the root-to-leaf paths in a binary tree | | | | |
| Find a root to leaf path with specified sum | | | | |
| Compute the *k*th node in an inorder traversal | | | | |
| Compute the successor | | | | |
| Implement an inorder traversal with *O(1)* space | | | | |
| Reconstruct a binary tree from traversal data | | | | |
| Reconstruct a binary tree from a preorder traversal with markers | | | | |
| Form a linked list from the leaves of a binary tree | | | | |
| Compute the exterior of a binary tree | | | | |
| Compute the right sibling tree | | | | |
| Implement locking in a binary tree | | | | |
Heaps
-----
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Merge sorted files | | | | |
| Sort an increasing-decreasing array | | | | |
| Sort an almost-sorted array | | | | |
| Compute the *k* closest stars | | | | |
| Compute the median of online data | | | | |
| Compute the *k* largest elements in a max-heap | | | | |
| Implement a stack API using a heap | | | | |
Searching
---------
### Binary Search
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Search a sorted array for first occurrence of *k* | | | | |
| Search a sorted array for the first element greater than *k* | | | | |
| Search a sorted array for entry equal to its index | | | | |
| Search a cyclically sorted array | | | | |
| Compute the integer square root | | | | |
| Compute the real square root | | | | |
### Generalized Search
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Search in a 2D sorted array | Solved | 1hr | Binary Search | Apr 30 |
| Find the min and max simultaneously | | | | |
| Find the *k*th largest element | | | | |
| Compute the optimum mailbox placement | | | | |
| Find the missing IP address | | | | |
| Find the duplicate and missing elements | | | | |
Hash Tables
-----------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Partition into anagrams | | | | |
| Solved for palindromic permutations | | | | |
| Is an anonymous letter constructible? | | | | |
| Implement an ISBN cache | | | | |
| Compute the LCA, optimizing for close ancestors | | | | |
| Compute the *k* most frequent queries | | | | |
| Find the nearest repeated entries in an array | | | | |
| Find the smallest subarray covering all values | | | | |
| Find smallest subarray sequentially covering all values | | | | |
| Find the longest subarray with distinct entries | | | | |
| Find the length of a longest contained range | | | | |
| Compute the average of the top three scores | | | | |
| Compute all string decompositions | | | | |
| Find a highest affinity pair | | | | |
| Solved the Collatz conjecture | | | | |
| Implement a hash function for chess | | | | |
Sorting
-------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Compute the intersection of two sorted arrays | | | | |
| Implement mergesort in-place | | | | |
| Count the frequencies of characters in a sentence | | | | |
| Find unique elements | | | | |
| Render a calendar | | | | |
| Sets of disjoint intervals | | | | |
| Compute the union of intervals | | | | |
| Partitioning and sorting an array with many repeated entries | | | | |
| Team photo day—1 | | | | |
| Implement a fast sorting algorithm for lists | | | | |
| Compute a salary threshold | | | | |
Binary Search Trees
-------------------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Solved if a binary tree satisfies the BST property | | | | |
| Find the first occurrence of a key in a BST | | | | |
| Find the first key larger than a given value in a BST | | | | |
| Find the *k* largest elements in a BST | | | | |
| Compute the LCA in a BST | | | | |
| Reconstruct a BST from traversal data | | | | |
| Find the closest entries in three sorted arrays | | | | |
| Enumerate numbers of the form *a + b√2* | | | | |
| The most visited pages problem | | | | |
| Build a minimum height BST from a sorted array | | | | |
| Insertion and deletion in a BST | | | | |
| Solved if three BST nodes are totally ordered | | | | |
| The range lookup problem | | | | |
| Add credits | | | | |
| Count the number of entries in an interval | | | | |
Recursion
---------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| The Tower of Hanoi problem | | | | |
| Generate all nonattacking placements of *n*-Queens | | | | |
| Generate permutations | | | | |
| Generate the power set | | | | |
| Generate all subsets of size *k* | | | | |
| Generate strings of matched parens | | | | |
| Generate palindromic decompositions | | | | |
| Generate binary trees | | | | |
| Implement a Sudoku solver | | | | |
| Compute a Gray code | | | | |
| Compute the diameter of a tree | | | | |
Dynamic Programming
-------------------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Count the number of score combinations | | | | |
| Compute the Levenshtein distance | | | | |
| Count the number of ways to traverse a 2D array | | | | |
| Plan a fishing trip | | | | |
| Search for a sequence in a 2D array | | | | |
| The knapsack problem | | | | |
| Divide the spoils fairly | | | | |
| The **bedbathandbeyond.com** problem | | | | |
| Find the minimum weight path in a triangle | | | | |
| Pick up coins for maximum gain | | | | |
| Count the number of moves to climb stairs | | | | |
| Compute the probability of a Republican majority | | | | |
| The pretty printing problem | | | | |
| Find the longest nondecreasing subsequence | | | | |
Greedy Algorithms and Invariants
--------------------------------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Implement Huffman coding | | | | |
| Compute an optimum assignment of tasks | | | | |
| Implement a schedule which minimizes waiting time | | | | |
| The interval covering problem | | | | |
| The 3-sum problem | | | | |
| Find the majority element | | | | |
| The gasup problem | | | | |
| Compute the maximum water trapped by a pair of vertical lines | | | | |
| Compute the largest rectangle under the skyline | | | | |
Graphs
------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Identify the celebrity | | | | |
| Search a maze | | |
| Paint a Boolean matrix | | | | |
| Compute enclosed regions | | | | |
| Degrees of connectedness—1 | | | | |
| Clone a graph | | | | |
| Making wired connections | | | | |
| Transform one string to another | | | | |
| The shorSolved straight-line program for *x^n* | | | | |
| Team photo day—2 | | | | |
| Compute a shorSolved path with fewest edges | | | | |
Parallel Computing
------------------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Implement caching for a multithreaded dictionary | | | | |
| Analyze two unsynchronized interleaved threads | | | | |
| Implement synchronization for two interleaving threads | | | | |
| Implement a thread pool | | | | |
| Implement asynchronous callbacks | | | | |
| Implement a Timer class | | | | |
| The readers-writers problem | | | | |
| The readers-writers problem with write preference | | | | |
| Solved the Collatz conjecture in parallel | | | | |
| Design TeraSort and PetaSort | | | | |
| Implement distributed throttling | | | | |
Design Problems
---------------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Design a spell checker | | | | |
| Design a solution to the stemming problem | | | | |
| Plagiarism detector | | | | |
| Pair users by attributes | | | | |
| Design a system for detecting copyright infringement | | | | |
| Design *TEX* | | | | |
| Design a search engine | | | | |
| Implement PageRank | | | | |
| Design a scalable priority system | | | | |
| Create photomosaics | | | | |
| Implement Mileage Run | | | | |
| Implement Connexus | | | | |
| Design an online advertising system | | | | |
| Design a recommendation system | | | | |
| Design an optimized way of distributing large files | | | | |
| Design the World Wide Web | | | | |
| Estimate the hardware cost of a photo sharing app | | | | |
Honors Class
------------
| Problem | Solved | Time To Solve | Solved Multiple Ways | Last Attempted | Comments
|--------------------------------------------------------------------------|:------------:|:-------:|
| Compute the greaSolved common divisor | | | | |
| Find the first missing positive entry | | | | |
| Buy and sell a stock *k* times | | | | |
| Compute the maximum product of all entries but one | | | | |
| Compute the longest contiguous increasing subarray | | | | |
| Rotate an array | | | | |
| Identify positions attacked by rooks | | | | |
| Justify text | | | | |
| Reverse sublists *k* at a time | | | | |
| Implement list zipping | | | | |
| Copy a postings list | | | | |
| Compute the median of a sorted circular linked list | | | | |
| Compute the longest substring with matching parens | | | | |
| Compute the maximum of a sliding window | | | | |
| Implement preorder and postorder traversals without recursion | | | | |
| Compute fair bonuses | | | | |
| Find *k* elements closest to the median | | | | |
| Search a sorted array of unknown length | | | | |
| Search in two sorted arrays | | | | |
| Find the *k*th largest element—large *n*, small *k* | | | | |
| Find an element that appears only once | | | | |
| Find the line through the most points | | | | |
| Find the shorSolved unique prefix | | | | |
| Compute the smallest nonconstructible change | | | | |
| Find the most visited pages in a window | | | | |
| Convert a sorted doubly linked list into a BST | | | | |
| Convert a BST to a sorted doubly linked list | | | | |
| Merge two BSTs | | | | |
| Solved if a binary tree is an almost BST | | | | |
| The view from above | | | | |
| Searching a min-first BST | | | | |
| Implement regular expression matching | | | | |
| Synthesize an expression | | | | |
| Count inversions | | | | |
| Draw the skyline | | | | |
| Find the two closest points | | | | |
| Measure with defective jugs | | | | |
| Compute the maximum subarray sum in a circular array | | | | |
| Determine the critical height | | | | |
| Voltage selection in a logic circuit | | | | |
| Find the maximum 2D subarray | | | | |
| Trapping water | | | | |
| Load balancing | | | | |
| Search for a pair-sum in an abs-sorted array | | | | |
| The heavy hitter problem | | | | |
| Find the longest subarray whose sum ≤ *k* | | | | |
| Degrees of connectedness—2 | | | | |
| Compute a minimum delay schedule, unlimited resources | | | | |
| Road network | | | | |
| Solved if arbitrage is possible | | | | |
| The readers-writers problem with fairness | | | | |
| Implement a producer-consumer queue | | | | |
------------------------
**Want to Revisit**
- binary search
- longest common subsequence
------------------------
**Classic Problems**
- lowest common ancestor
- is same tree
- is subtree
- invert binary tree
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