update

Author: yennanliu

README.md

@@ -313,7 +313,7 @@
 454| [4Sum II](https://leetcode.com/problems/4sum-ii/) | [Python ](./leetcode_python/Hash_table/4sum-ii.py)  | _O(n^2)_ | _O(n^2)_ | Medium | check LC 018 4SUM, `trick`, `basic`, `amazon`| AGAIN**
 470| [Implement Rand10() Using Rand7()](https://leetcode.com/problems/implement-rand10-using-rand7/) |  [Python ](./leetcode_python/Hash_table/implement-rand10-using-rand7.py)  | _O(1)_  | _O(1)_  | Medium  |`trick`, `google`| AGAIN** (3)
 473| [Matchsticks to Square](https://leetcode.com/problems/matchsticks-to-square/) |  [Python ](./leetcode_python/Hash_table/matchsticks-to-square.py)  | _O(n * s * 2^n)_  | _O(n * (2^n + s))_  | Medium  |`complex`,`hard`| AGAIN (not start)
-523| [Continuous Subarray Sum](https://leetcode.com/problems/continuous-subarray-sum/) | [Python ](./leetcode_python/Hash_table/continuous-subarray-sum.py) | _O(n)_  | _O(k)_ | Medium |sub array sum, check `# 560 Subarray Sum Equals K`,`good trick`,`substring`, `hash table`,`AGAIN`,`M$`,`fb`| AGAIN************** (9) (MUST)
+523| [Continuous Subarray Sum](https://leetcode.com/problems/continuous-subarray-sum/) | [Python ](./leetcode_python/Hash_table/continuous-subarray-sum.py) | _O(n)_  | _O(k)_ | Medium |sub array sum, check `# 560 Subarray Sum Equals K`,`good trick`,`substring`, `hash table`,`AGAIN`,`M$`,`fb`, `apple`| AGAIN************** (9) (MUST)
 525| [Contiguous Array](https://leetcode.com/problems/contiguous-array/) |  [python](./leetcode_python/Hash_table/contiguous-array.py)  | _O(n)_ | _O(n)_ | Medium     | sub-array sum, `good basic`, `array`, `hashmap`, `cache`, `AGAIN`, `fb`, amazon| AGAIN**************** (10) (MUST)
 532| [K-diff Pairs in an Array](https://leetcode.com/problems/k-diff-pairs-in-an-array/) | [Python ](./leetcode_python/Hash_table/k-diff-pairs-in-an-array.py)  | _O(n)_ | _O(n)_ | Medium |hash table, `basic`, `collections.Counter()`,`a-b =k -> a = k + b `, `amazon`| AGAIN********** (6)
 554| [Brick Wall](https://leetcode.com/problems/brick-wall/) | [Python ](./leetcode_python/Hash_table/brick-wall.py) | _O(n)_ | _O(m)_ | Medium  |`trick`,`hash map`, `bloomberg`, `fb` | OK***** (but again)(5)

doc/progress.md

@@ -63,6 +63,8 @@
 			- p.363 -
 	- system design book - an insider's guide (GOTOP)
 		- 1st read (ok)
+		- 2nd read : note
+			- p.1 - p.35
 		- review/note
 
 # 2022-02-08

system_design/sys_design_faq.md

@@ -42,11 +42,20 @@
 - system design book (GOTOP) P.18
 
 ### 8) What's cache ?
+- system design book (GOTOP) P.11
 
 ### 9) What's DC (data center) ?
+- system design book (GOTOP) P.21
 
 ### 10) What's message queue ?
+- system design book (GOTOP) P.23
 
 ### 11) examples on metric, logging, automation ? what to track ? why ?
+- system design book (GOTOP) P.25
+- Metrics
+	- server (machine) level metric : CPU, memory, disk IO..
+	- integrated level metric : all DB layer, cache layer ...
+	- business metric : DAU, retention, income (e.g. GMV) ...
 
 ### 12) DB scaling out strategy ?
+- system design book (GOTOP) P.27