(1) Arrays & Hashing

No.	Name	Num solutions	Descriptions	Functions
1	Two Sum	1	HashMap	enumerate(),preMap = {}
36	Valid Sudoku	1	DefaultDictionary	defaultdict(), board[r][c]
49	Group Anagram	1	DefaultDictionary	defaultdict(), tuple(),count = [0] * 26
128	Longest Consecutive Sequence	1	set, length = y-n	set()
217	Contains Duplicate	4	Hashset	sort(), set()
238	Product of Array Except Self	2	-	-
242	Valid Anagram	3	HashMap	enumerate(), counter()
271	Encode and Decode Strings	1	Record the length of each string	str(len(i)) + "#" + i
347	Top K Frequent Elements	1	HashMap+Array	get(), items()
659	Split Array into Consecutive Subsequences	1	Hashmap	collections.Counter() or defaultdict(int)

Iterate the array in decending order: for i in range(len(freq)-1, 0, -1):

(2) Two pointers

No.	Name	Num solutions	Descriptions	Functions
11	Container With Most Water	1	2 pointers: Move the smaller one	-
15	3Sum	1	2 pointers + HashMap	sort(), enumerate(), append()
42	Trapping Rain Water	1	2 pointers:similar to No.11	-
125	Valid Palindrome	2	2 pointers	isalnum()
1470	Shuffle the Array	1	2 pointers	-

(3) Sliding Window

No.	Name	Num solutions	Descriptions	Functions
3	Longest Substring Without Repeating Characters	1	2 pointers + set: use while not if	charset = set()
121	Best Time to Buy and Sell Stock	1	2 pointers l=r	-
424	Longest Repeating Character Replacement	1	2 pointers + HashMap	count.get(s[r], 0)
567	Permutation in String	1	2 pointers + index	ord(), [0]* 26

(4) Stack

No.	Name	Num solutions	Descriptions	Functions
20	Valid Parentheses	1	Hashmap+Stack	stack = [], pop(), append()
22	Generate Parentheses	1	Stack+function	stack = []
150	Evaluate Reverse Polish Notation	1	Stack	pop(), append(), 5 cases
155	Min Stack	1	2* Stack	self.stack = [], pop(), append()
739	Daily Temperatures	1	stack((t,i))	res[stackC] = i - stackC, enumerate()

(5) Binary Search

No.	Name	Num solutions	Descriptions	Functions
33	Search in Rotated Sorted Array	1	2 sorted arrays	if nums[l] <= nums[m] on the left portion
34	Find First and Last Position of Element in Sorted Array	1	rightMargin & leftMargin	nums[r] == targetkeep searching
74	Search a 2D Matrix	2	newArray or ChooseRow	ows, cols = len(matrix), len(matrix[0])
704	Binary Search	1	2 Pointer	while l <= r, m = l + (r-l)// 2
875	Koko Eating Bananas	1	leftMargin	if totalTime <= h: k=m r=m-1, math.ceil() & //
StoO 53	Finding Numbers in Sorted Arrays I	1	rightMargin & leftMargin: !return potiner	length= rs - ls + 1

(6) Linked List

No.	Name	Num solutions	Descriptions	Functions
2	Add Two Numbers	1	carry = val // 10	v1 = l1.val if l1 else 0, l1 = l1.next if l1 else None
19	Remove Nth Node From End of List	1	reverse or locate the target node with l&r	dummy = ListNode(0,head)
138	Copy List with Random Pointer	1	hashmap: copy and link nodes	copy = Node(cur.val)
141	Linked List Cycle	1	fast&slow pointers	dummy = ListNode(0,head)
143	Reorder List	1	Medium, reverse+merge	-
287	Find the Duplicate Number	1	LIST, Floyd's : Cycle, fast+slow & slow+slow2	fast = nums[nums[fast]]

(7) Trees

No.	Name	Num solutions	Descriptions	Functions
98	Validate Binary Search Tree	1	iterate DFS	float("-inf")
100	Same Tree	1	reverse	self.isSameTree(p.left, q.left)
102	Binary Tree Level Order Traversal	1	BFS	collections.deque()
104	Maximum Depth of Binary Tree	3	Good question to learn DFS, BFS	stack[],deque(),append()on the right of queue,q.popleft()
105	Construct Binary Tree from Preorder and Inorder Traversal	1	building a tree basded on preorder and inorder	mid = inorder.index(preorder[0])
107	Binary Tree Level Order Traversal II	1	iterate + reverse	res[::-1]
110	Balanced Binary Tree	1 recursive DFS	[bool, height]	-
226	Invert Binary Tree	1	Switch right and left	self.invertTree()
230	Kth Smallest Element in a BST	1	Switch right and left	self.invertTree()
235	Lowest Common Ancestor of a Binary Search Tree	1	iterative	frist left then right
572	Subtree of Another Tree	1	Check same tree? subtree?	self.isSameTree()
543	Diameter of Binary Tree	1	Start form bottom, update height+diameter
1448	Count Good Nodes in Binary Tree	1	DFS	res = 1 if node.val >= maxVal else 0

(8) Heap

No.	Name	Num solutions	Descriptions	Functions
703	Kth Largest Element in a Stream	1	MinHeap, pop the smallest value -> return minHeap[0]	heapq.heapify()

heapq.heappop(): Pop and return the smallest item from the heap

heapq.heappush(): Push the value item onto the heap

heapq.heapify(x): Transform list x into a heap, in-place, in linear time.

(0) Others

No.	Name	Num solutions	Descriptions	Functions
9	Palindrome Number	3	String	invert = invert*10 + n//10
45	Jump Game II	1	Greedy	-
172	Factorial Trailing Zeroes	1	the number of factorial trailing 0 = //5	-
692	Top K Frequent Words	1	Samilar to 347	collections.Counter(), sorted(list, key=lamda x:(-cnt[x], x))[:k]
208	Implement Trie (Prefix Tree)	1	Trie	TrieNode()

Data Structures

Deque

· Doubly Ended Queue, suitable when need quicker append and pop operations from both the ends of the container.

Time: O(1) for pop and append operations

index(ele, beg, end), insert(i, a) a is the element, i is the position, remove()removes the first occurrence of the selected value

append() add on the right, appendleft(); pop() remove from the left, popleft()