XOR operation: 1. a XOR 0 = a 2. a XOR a = 0 3. a XOR b XOR a = (a XOR a) XOR b = 0 XOR b = b Reference: 1. https://leetcode.com/articles/single-number/

Its useful to sync a fork when contributing to open source project. Here is some tips: 1. make sure current folder is repo root. 2. add upstream link. Here, upstream could be any name git remote add upstream https://github.com/whoever/whatever.git 3. fetch content. ``git fetch upstream 4. goto your branch and merge (keep local changes) or rebase (do not keep) upstream branch. Reference: 1. https://stackoverflow.com/questions/7244321/how-do-i-update-a-github-forked-repository 2. https://help.github.com/en/articles/syncing-a-fork

624. Maximum Distance in Arrays 609. Find Duplicate File in System

http://hackjutsu.com/2016/01/20/Java%20Concurrency/

79. Word Search 212. Word Search II 92. Reverse Linked List II 160. Intersection of Two Linked Lists 287. Find the Duplicate Number 268. Missing Number 151. Reverse Words in a String 186. Reverse Words in a String II 215. Kth Largest Element in an Array 88. Merge Sorted Array 437. Path Sum III