update

Author: KaidiGuo

README.md

@@ -37,7 +37,13 @@ For question details, please go to [Leetcode](https://leetcode.com/problemset/al
    + Medium 146. LRU Cache -- `[collections.OrderedDict()]`--`[dict.popitem(last=False)]`
 
 4. [Sliding window](https://github.com/KaidiGuo/Algorithm-Exercises/tree/master/Linked%20List)
+
+   When to use sliding window?
+   Question like:
+   - "finding the substring that meet certain criterion."
+   ---
    + Medium 438 Find All Anagrams in a String
+   + Medium 209. Minimum Size Subarray Sum
 
 5. [Linked List](https://github.com/KaidiGuo/Algorithm-Exercises/tree/master/Linked%20List)
    + Medium 2 Add Two Numbers  -- `[carry = carry//10]`
@@ -48,12 +54,18 @@ For question details, please go to [Leetcode](https://leetcode.com/problemset/al
    + Medium 105 Construct Binary Tree from Preorder and Inorder Traversal  -- recursive
 
 7. [Two Pointers](https://github.com/KaidiGuo/Algorithm-Exercises/tree/master/Linked%20List)
+   
+   When to use two pointers?
+   Question like:
+   - "handle the list/string IN PLACE, I want to do something when elements meet certain criterion"
+   ---
    + Medium 763 Partition Labels
    + Easy 27. Remove Element
    + Easy 283. Move Zeroes
    + Easy 844. Backspace String Compare
+   + Easy 977. Squares of a Sorted Array
 
-8. [Array](https://github.com/KaidiGuo/Algorithm-Exercises/tree/master/Linked%20List)
+8. [Array-binary search](https://github.com/KaidiGuo/Algorithm-Exercises/tree/master/Linked%20List)
    + Easy 937. Reorder Data in Log Files -- `[list.sort(reverse=False, key=(key1,key2))]`
    + Easy 704. Binary Search  -- `[Binary Search]`
    + Easy 69. Sqrt(x)  -- `[Binary Search]`