update readme, cheatsheet, add code comment

Author: yennanliu

README.md

@@ -1153,7 +1153,7 @@
 
 |  #  | Title           |  Solution       |  Time           | Space           | Difficulty    | Status          | Note| 
 |-----|---------------- | --------------- | --------------- | --------------- | ------------- |--------------|-----|
-269 |[Alien Dictionary](https://leetcode.com/problems/alien-dictionary/)  | [Python](./leetcode_python/Graph/alien-dictionary.py), [Java](./leetcode_java/src/main/java/LeetCodeJava/Graph/AlienDictionary.java) | | | Hard| Curated Top 75, dfs, bfs, topology sort, `fb`, google, m$, airbnb, uber, amazon |  AGAIN******** (4) (not start)
+269 |[Alien Dictionary](https://leetcode.com/problems/alien-dictionary/)  | [Python](./leetcode_python/Graph/alien-dictionary.py), [Java](./leetcode_java/src/main/java/LeetCodeJava/Graph/AlienDictionary.java) | | | Hard| Curated Top 75, dfs, bfs, Topologicals sort, `fb`, google, m$, airbnb, uber, amazon |  AGAIN******** (4) (not start)
 323| [Number of Connected Components in an Undirected Graph](https://leetcode.com/problems/number-of-connected-components-in-an-undirected-graph/) | [Python ](./leetcode_python/Graph/number-of-connected-components-in-an-undirected-graph.py),  [Java ](./leetcode_java/src/main/java/LeetCodeJava/Graph/NumberOfConnectedComponentsUndirectedGraph.java) | _O(n)_ | _O(n)_| Medium  | Curated Top 75, LC 547, 🔒 , dfs, Union Find, `graph`, `linkedin`, `amazon`, fb, google, m$, linkedin|  AGAIN************** (5) (MUST)
 959 | [Regions Cut By Slashes](https://leetcode.com/problems/regions-cut-by-slashes/)  | [Python](./leetcode_python/Graph/regions-cut-by-slashes.py) | _O(n^2)_| _O(n^2)_| Medium | Union Find | 
 1135 | [Connecting Cities With Minimum Cost](https://leetcode.com/problems/connecting-cities-with-minimum-cost/)  | [Python](./leetcode_python/Graph/connecting-cities-with-minimum-cost.py) | _O(n^2)_| _O(n^2)_| Medium | union find, Kruskal, prime, graph, amazon | AGAIN (not start)

doc/cheatsheet/topology_sorting.md

@@ -18,6 +18,8 @@
     - LC 444
 - Alien Dictionary
     - LC 269
+- Others
+    - LC 666, 802
 
 ### 0-2) Pattern
 ```

leetcode_java/src/main/java/LeetCodeJava/Graph/AlienDictionary.java

@@ -5,6 +5,56 @@
 
 import java.util.*;
 
+/**
+ * 269. Alien Dictionary
+ *
+ * There is a new alien language which uses the latin alphabet.
+ * However, the order among letters are unknown to you.
+ * You receive a list of non-empty words from the dictionary,
+ * where words are sorted lexicographically by the rules of this new language.
+ * Derive the order of letters in this language.
+ *
+ * Example 1:
+ *
+ * Input:
+ * [
+ *   "wrt",
+ *   "wrf",
+ *   "er",
+ *   "ett",
+ *   "rftt"
+ * ]
+ *
+ * Output: "wertf"
+ * Example 2:
+ *
+ * Input:
+ * [
+ *   "z",
+ *   "x"
+ * ]
+ *
+ * Output: "zx"
+ * Example 3:
+ *
+ * Input:
+ * [
+ *   "z",
+ *   "x",
+ *   "z"
+ * ]
+ *
+ * Output: ""
+ *
+ * Explanation: The order is invalid, so return "".
+ * Note:
+ *
+ * You may assume all letters are in lowercase.
+ * You may assume that if a is a prefix of b, then a must appear before b in the given dictionary.
+ * If the order is invalid, return an empty string.
+ * There may be multiple valid order of letters, return any one of them is fine.
+ *
+ */
 public class AlienDictionary {
 
     // V0