LeetCode solutions.

Always try to optimize time complexity O(n^2) -> O(nlogn) or O(n) in most cases.

Python Tips:

1. Dictionary build-in method keys() will not return a list object. To get a list of keys, need try list(newdict.keys()). This will convert the dict_keys object to a list.
2. Dictionary (Hash Table) should be considered in priority to deal with the duplicated information or the mapping relationship.
3. For Linked List problem, using/creating a dummy head is usually needed.
4. For Linked List problem, the middle element of a list can be found by using a slow and a fast pointers together.

Java Tips:

1. The ASCII values of '`' = 96, 'a' = 97, 'z' = 122, '{' = 123. The full ASCII table can be found at: https://www.cs.cmu.edu/~pattis/15-1XX/common/handouts/ascii.html

Data Structure Implementation:

1. An implementation of Trie can be found in 720. Longest Word in Dictionary
2. An implementation of Max Heap can be found in 692. Top K Frequent Words

Array Problems:

1. Using 2 pointers.
2. Or using Priority Queue (Heap) if some kinds of ordering is required.
3. Regarding circular array, possible solution is: -- concatenate the array with its copy to get a new array with double length. But cost extra space. -- Or, use larger index range like 0 < i < 2n with mod operation i%n, here the n is the length of array.

List Problems:

1. Using two pointers (slow pointer and fast pointer) to locate the middle element of the list.

Memoization:

1. For a tree-like data structure, probably need a Hash table such as HashMap<TreeNode, Integer> to store the calculated values for Java implementation.

Sorting:

1. If the number of data is not big and related to integer, could using Counting Sort for better Time Complexity.
2. TreeMap is a choice when handle sorting of large number data.

Binary Search:

1. Use 'index space' as searching space in 1-dimension data like array
2. Use 'number range space' as searching space in 2-dimension data like matrix

Dynamic Programming:

1. the Math.max() and Math.min() are often used in Java solutions
2. a local extremum (optimal) value is always used
3. a matrix dp[i][j] is used to store result in most cases

Bit Manipulation:
Use Column Operation

Interval Problem: Use TreeMap with its lowerKey() or floorKey() and higherKey() or ceilingKey()

Using HashMap to reduce Time Complexity:
0560. Subarray Sum Equals K

Sort Time Stamp with TreeMap:
0253. Meeting Rooms II
0731. My Calendar II
1094. Car Pooling

Boyer-Moore Majority Vote algorithm
0229. Majority Element II

Morris Traversal 0099. Recover Binary Search Tree

Good discussion and explanation:

1. Approach the problem using the "trial and error" algorithm:
   https://leetcode.com/problems/find-k-th-smallest-pair-distance/discuss/109082/Approach-the-problem-using-the-%22trial-and-error%22-algorithm
2. Summary of solutions for problems "reducible" to LeetCode 378:
   https://leetcode.com/problems/k-th-smallest-prime-fraction/discuss/115819/Summary-of-solutions-for-problems-%22reducible%22-to-LeetCode-378

Heuristic Solutions:

14. Longest Common Prefix
15. Maximum Subarray
16. Best Time to Buy and Sell Stock III
17. Linked List Cycle
18. Linked List Cycle II
19. Intersection of Two Linked Lists
20. Majority Element
21. Palindrome Linked List
22. Range Sum Query 2D - Immutable