LeetCode

#	Title	Difficulty	Tags	Notes
1	Two Sum	Easy	Hash Table	build hash table while searching
2	Add Two Numbers	Medium	Linked List, Math	save a dummy head for output
3	Longest Substring Without Repeating Characters	Medium	Hash Table, Two Pointers, String	update the max length when traversing with the right pointer, move left pointer to ensure non-repeating
4	Median of Two Sorted Arrays	Hard	Binary Search	find the largest numbers on the left halves and smallest numbers on the right halves in each array, then use the formula median = 0.5 * (max(L1, L2) + min(R1, R2))
7	Reverse Integer	Easy	Math	test overflow
8	String to Integer (atoi)	Easy	String, Math	remove leading white spaces, test overflow
15	3Sum	Medium	Two Pointers	sort first, convert into two sum problem then approach from both sides
16	3Sum Closest	Medium	Two Pointers	calculate the absolute value
18	4Sum	Medium	Two Pointers	4Sum -> 3Sum -> 2Sum
28	Implement strStr()	Easy	KMP Algorithm, String	traverse all the possible starting points and see if the following characters in haystack match those of needle
30	Substring with Concatenation of All Words	Hard	Hash Table, String	build one hash table with the goal, update the other while searching
43	Multiply Strings	Medium	String, Math	use an array of integers to store results from multiplying single digit serparately
65	Valid Number	Hard	String, Math	use two flags for '.' and 'e', check the number before and after 'e' for output
66	Plus One	Easy	Math	push back a '0' instead of add a '1' at the front to save time
67	Add Binary	Easy	String, Math	add if either of a, b, or carry is not empty
76	Minimum Window Substring	Hard	Hash Table, Two Pointers, String	build hash table, move right pointer to match, move left pointer to find minimum
155	Min Stack	Easy	Stack, Design	use two stacks to implement, keep the current minimum on the top
167	Two Sum II - Input array is sorted	Medium	Two Pointers	approach from both sides
239	Sliding Window Maximum	Hard	Queue	use a double-ended queue (deque) to store the potential maximum
306	Additive Number	Medium	String, Math	handle leading zero cases and recursively test if it is additive
371	Sum of Two Integers	Easy	Bit Manipulation	use XOR to handle '0 + 0', '0 + 1', '1 + 0', use AND then left shift to handle '1 + 1'
415	Add Strings	Easy	String, Math	loop if either of num1, num2, or carry is not empty
445	Add Two Numbers II	Medium	Linked List, Math	record a post representing the first location that could be affected by carry, then add 1 to the post, set the following to 0
454	4Sum II	Medium	Hash Table	use hash table for search purpose