Leetcode Pattern 3 | Backtracking

Here is the article.

Algorithms:

Leetcode 78. Subsets
90. Subsets II
39. Combination sum
40. Combination Sum II
51. N-Queens

My practice

78. Subsets - submission in May 2019
90. Subsets II - no submission
39. Combination sum - a year ago, two year ago
40. Combination sum II - a year ago, two year ago

78
Here is my post on leetcode discuss I wrote on June 22, 2019.

39
I wrote a post to share on June 22, 2019 based on practice in 2018. Here is the link.
I wrote a post to share on June 22, 2019 based on practice in 2017. Here is the link.

40.
Here is my discussion link for practice in 2018.
Here is my discussion link for practice in 2017.

51
Here is my post written on June 22, 2019. The code was submitted on January 26, 2016. I missed 2016. I got my first Amazon onsite in the city of Vancouver, Canada. I turned 50 years old in October, 2016.

90 
Here is my practice, first one in 2019. The code should be simplified. There are some duplicated code. 

Here is C# practice using recursive solution, I studied the code written by an Amazon engineer.