# 99 Elm Problems/Problems by Category
### Lists

* Problem 1: Find the last element of a list
* Problem 2: Find the next to last element of a list
* Problem 3: Find the n-th element of a list
* Problem 4: Find the number of elements of a list
* Problem 5: Reverse a list
* Problem 6: Determine if a list is a palindrome
* Problem 7: Flatten a nested list structure
* Problem 8: Eliminate consecutive duplicates of list elements
* Problem 9: Pack consecutive duplicates of list elements into sublists
* Problem 10: Run length encode a list of tuples

### Lists, continued

* Problem 11: Run-length encoding
* Problem 12: Run-length decoding
* Problem 14: Duplicate the elements
* Problem 15: Repeat each element
* Problem 16: Drop every N'th element
* Problem 17: Split a list into two parts
* Problem 18: Extract a sublist
* Problem 19: Rotate a list N places
* Problem 20: Remove the nth element

### Lists again

* Problem 21: Insert an element
* Problem 22: Create a range of integers
* Problem 23: Randomly selected elements
* Problem 24: Randomly select from a set
* Problem 25: Generate a random permutation
* Problem 26: Generate combinations of a set
* Problem 27: Group elements into disjoint subsets
* Problem 28: Sort a list of lists

### Arithmetic

* Problem 31: Check for prime
* Problem 32: Find greatest common divisor
* Problem 33: Check for coprime
* Problem 34: Calculate Euler's totient function
* Problem 35: Find prime factors
* Problem 36: List the prime factors
* Problem 37: Calculate Euler's totient function using prime factors
* Problem 39: List prime numbers within a range
* Problem 40: Goldbach's conjecture
* Problem 41: Search a range of numbers

### Logic and Codes

* Problem 46: Define functions for logical operations
* Problem 47: Build truth tables for logical expressions
* Problem 48: Build truth tables for any logical expressions
* Problem 49: Gray codes
* Problem 50: Huffman codes

### Binary Trees

* Problem 55: Construct a balanced binary tree
* Problem 56: Construct a symmetric binary tree
* Problem 57: Construct a binary search tree
* Problem 58: Generate and test paradigm
* Problem 59: Construct a height balanced binary tree
* Problem 60: Construct a height-balanced binary tree of N nodes

### Binary Trees, continued

* Problem 61: Count the leaves
* Problem 62: Collect the internal nodes
* Problem 63: Construct a complete binary tree
* Problem 64: Plot node coordinates 1
* Problem 65: Plot node coordinates 2
* Problem 66: Plot node coordinates 3
* Problem 67: Represent a tree as a string
* Problem 68: Generate an ordered sequence
* Problem 69: Represent a tree with Dot Notation

### Multi-way Trees

* Problem 70: Count the nodes of a multiway tree
* Problem 71: Internal path of a multiway tree
* Problem 72: Bottom-up sequence of a multiway tree
* Problem 73: Lisp representation of a multiway tree

### Graphs

* Problem 80: Convert graphs representations
* Problem 81: Find all cyclic paths
* Problem 82: Find closed paths
* Problem 83: Find spanning trees
* Problem 84: Find the minimal spanning tree
* Problem 85: Determine if two graphs are isomorphic
* Problem 86: Calculate degree of a node
* Problem 87: Depth-first order graph traversal
* Problem 88: Connected components
* Problem 89: Check for bipartite graph

### Puzzles

* Problem 90: Eight Queens
* Problem 91: The Knight's Tour
* Problem 92: Graceful Labeling
* Problem 93: Inserting operators
* Problem 94: Generate regular graphs
* Problem 95: Numbers to text
* Problem 96: Syntax checker
* Problem 97: Sudoku solver
* Problem 98: Nonogram solver
* Problem 99: Elm Problems: Crossword puzzle solver