Advent of Code Solutions in Nim

* denotes tricky problems

2020

Day	Tags	Memo
25	implementation
24	hexagon, simulation	See 2017 day 11
23*	simulation, table	part2: use hash table to implement circular linked list
22	simulation, deque
21	set
20*	implementation	borders are unique; monsters won't overlap
19*	dfs, grammar	part2: brute force rules
18	expression, stack	Shunting yard algorithm
17	simulation
16	implementation, set
15	simulation
14	implementation
13	math, chinese remainder theorem	implement crt
12	implementation	turning degrees are multiples of 90
11	simulation
10	implementation, sort, counter, dp
9	implementation, prefix sum, bisect
8	simulation, brute force
7	regex, graph, dfs
6	implementation, set
5	implementation
4	implementation, regex
3	implementation
2	implementation
1	two pointers

2019

Day	Tags	Memo
25	simulation, dfs, interactive, brute force	solve day 9 first; the map is a tree not a grid
24	simulation
23	simulation	solve day 9 first
22*	matrix, modint, bigint	implement matrix, modint
21*	simulation, brute force	solve day 9 first; only needs an additional "H"; slow
20	parsing, bfs	labels are read either left-to-right or top-to-bottom; part2: compress states by building graph
19	simulation, brute force	solve day 9 first
18*	bfs, bitmask	part1: same problem on leetcode; part2: compress states by skipping empty cells; slow
17	simulation, dfs, brute force, rle	solve day 9 first
16*	simulation, suffix sum	part2: the offset is guaranteed at second half
15*	simulation, backtracking, bfs	solve day 9 first; get a map first
14	graph, topological sorting, bisect
13	simulation	solve day 9 first
12	simulation, period	part2: find peroid for each axis independently
11	simulation	solve day 9 first
10	math, simulation
9	implementation	solve day 5 first
8	implementation
7	simulation, permutation	solve day 5 first
6	tree, dfs
5	simulation	solve day 2 first
4	brute force
3	simulation, hash
2	implementation, brute force
1	implementation

2018

Day	Tags	Memo
25	disjoint set
24	simulation	look out for draw states that would make combats unable to proceed
23*	graph, clique, bisect	add a new bot at the origin, and find the shortest range of it that would make it into a largest clique
22	implementation, bfs
21	reverse engineering, period	solve day 19 first
20*	dfs, bfs	regex implementation
19	simulation, reverse engineering	sum of divisors
18	simulation, grid, period
17*	simulation, dfs
16	backtracking, simulation
15*	simulation, bfs	if a unit could not find a living oppenent in its turn, the round is incomplete and not counted for
14	simulation
13	simulation
12	simulation, period
11	implementation, grid, integral image, brute force
10	simulation	run simulation until the bounding box of points is small enough
9	simulation, linked list
8	dfs
7	graph, topological sorting, priority queue
6	grid, flood fill
5	string, simulation, stack, brute force
4	implementation
3	grid	implement Imos
2	implementation, string
1	implementation, hash

2017

Day	Tags	Memo
25	simulation, parsing
24	dfs
23	simulation, reverse engineering	part2: check prime
22	simulation, hash
21	simulation	Glider
20	simulation, math	part2: settles under 50 secs
19	simulation
18	simulation, coroutine
17	simulation
16	simulation, period
15	implementation, brute force
14	implementation, flood fill	solve day 10 first
13	math, brute force	similar to 2016 day 15
12	graph, disjoint set	implement Disjoint-set/Union-find
11	math	Hexagonal Grids
10	string, implementation
9	string, parsing
8	simulation
7	graph, dfs	nodes with two subtrees are always balanced
6	simulation, hash
5	simulation
4	string, implementation
3	math, simulation
2	implementation, brute force
1	string, implementation

2016

Day	Tags	Memo
25	simulation, vm, brute force, reverse engineering	integer in binary: reverse engineered
24	grid, bfs
23	simulation, vm, reverse engineering	part2: factorial
22*	grid, bfs	part2: move only the empty node and the target node
21	string, simulation, brute force, permutation
20	simulation
19	math, simulation	implement OrderedList
18	string, implementation
17	bfs, brute force
16	string, implementation
15	math, brute force
14	string, sliding window
13	bfs
12	simulation, vm, reverse engineering	reverse engineered
11*	bfs	items with same types are equivalent, count number of items for each type only
10	simulation
9	string, simulation, dfs
8	simulation, grid
7	string, implementation
6	implementation
5	brute force, implementation
4	implementation
3	math
2	simulation
1	simulation

2015

Day	Tags	Memo
25	math, fastExp
24	brute force, dfs
23	simulation, vm, reverse engineering	Collatz conjecture
22	simulation, bfs
21	brute force, math
20	math
19*	dfs	part2: reduce target string from back to front using reversed rules
18	simulation, grid
17	brute force, bitops
16	brute force
15	dfs, brute force
14	math, simulation
13	brute force, permutation
12	dfs
11	string, brute force
10	simulation
9	graph, brute force, permutation
8	string
7	dfs, memoization
6	simulation, grid
5	string, regex
4	brute force, implementation
3	simulation
2	math
1	stack