Add AtCoder/ABC157D to challenges

Author: xirc

lib/cpalgo/ds/README.md

@@ -6,6 +6,7 @@ a.k.a Union Find Tree
 
 ### Challenges
 - [道路の老朽化対策について - AtCoder ABC040D](https://atcoder.jp/contests/abc040/tasks/abc040_d)
+- [D - Friend Suggestions](https://atcoder.jp/contests/abc157/tasks/abc157_d)
 
 ## Binary Indexed Tree
 

lib/cpalgo/graph/README.md

@@ -29,7 +29,14 @@ We can find all articulation points of a given graph in `O(V + E)`.
 - [AtCoder | E - League](https://atcoder.jp/contests/abc139/tasks/abc139_e)
 
 
-## Strongly Connected Components
+## Connected Components (Undirected Graph)
+🚧
+
+### Challenges
+- [D - Friend Suggestions](https://atcoder.jp/contests/abc157/tasks/abc157_d)
+
+
+## Strongly Connected Components (Directed Graph)
 🚧