204. https://assets.leetcode.com/static_assets/public/images/solutions/Sieve_of_Eratosthenes_animation.gif

205. Battleships in a Board https://blog.csdn.net/camellhf/article/details/52871104

206. Custom Sort Strin https://leetcode.com/problems/custom-sort-string/discuss/116615/Java-5-ms-10-line-counting-solution-with-comment

207. Maximum XOR of Two Numbers in an Array https://kingsfish.github.io/2017/12/15/Leetcode-421-Maximum-XOR-of-Two-Numbers-in-an-Array/

208. Linked List Cycle II https://leetcode.com/problems/linked-list-cycle-ii/discuss/44793/O(n)-solution-by-using-two-pointers-without-change-anything:wq

// a b // start ------->-------->meeting // | | // <---------- // c // assume fast and slow meets at k steps // k=a+b+r1(b+c) slow runs r1 cycles // 2k=a+b+r2(b+c) fast runs r2 cycles // 2k=a+b+r2(b+c)=2a+2b+2r1(b+c) // (b+c)(r2-2r1)=a+b => (b+c)n=a+b // a=(n-1)b+nc=(n-1)(b+c)+c which means when slow moves (n-1) cycles and c, start moves a

31. https://www.cnblogs.com/yuzhangcmu/p/4221998.html

32. Best Time to Buy and Sell Stock II https://hackmd.io/pdb3S3syTpWUo43VYaYvBA

33. https://leetcode.com/problems/candy/discuss/42774/Very-Simple-Java-Solution-with-detail-explanation (grahp on note book)

34. http://www.yikanggao.com/blog/2017/07/%E8%AE%A4%E8%AF%86%E5%8D%95%E8%B0%83%E9%98%9F%E5%88%97.html https://www.tutorialspoint.com/java/util/java_util_arraydeque.htm

35. 1.https://blog.csdn.net/baidu_23318869/article/details/50386323 2.https://blog.csdn.net/qq508618087/article/details/50517503

36. qustion 322 and question 279 are done by the same way

37. https://blog.csdn.net/fuxuemingzhu/article/details/79537797

38. https://kingsfish.github.io/2017/12/15/Leetcode-421-Maximum-XOR-of-Two-Numbers-in-an-Array/

Knowledge

1. change 1 bit from 0 (1) to 1 (0) 1.1 xxxxxxxxxxxxx0xxx ^ 00000000000001000

---

xxxxxxxxxxxxx1xxx

1.2 xxxxxxxxxxxxx1xxx ^ 00000000000001000

xxxxxxxxxxxxx0xxx

1.2 change 2 bits from 0s (1s) to 1s (0s) xxxx1xxxxxxxx0xxx ^ 00001000000001000

xxxx0xxxxxxxx1xxx

2. if a ^ b = x -> a ^ x = b; -> x ^ a = b; -> b ^ x = a; -> x ^ b = a;

3. a ^ a = 0 b ^ 0 = b b ^ a ^ a = b

4. Wiggle Subsequence https://hackmd.io/HF9EOCxYSdmaQrgSBE-19Q

5. similar to question 1 (2 sum)

6. https://stackoverflow.com/questions/13032116/trie-complexity-and-searching