Update README.md

Added a few more questions which can be solved using this technique.

Author: amulyagaur

Graph/Sack/README.md

@@ -4,6 +4,10 @@ A concept introduced in the algorithm is that of heavy son, in which the heavier
 Some problems that can be solved with the Sack algorithm:
 * [Lomsat gelral - Codeforces](https://codeforces.com/problemset/problem/600/E)
 * [Tree Requests - Codeforces](https://codeforces.com/problemset/problem/570/D)
+* [Coloring Tree - HackerRank](https://www.hackerrank.com/challenges/coloring-tree/problem)
+* [Hedwig The Smart Bird - Codechef](https://www.codechef.com/problems/HEDWIG)
+* [Summing in a tree - HackerRank](https://www.hackerrank.com/contests/101hack47/challenges/summing-in-a-tree)
+* [Treedif - VJudge](https://vjudge.net/problem/SGU-507)
 
 Troubleshooting the above using sack:
 * [Lomsat Gelral Solution](https://notepad.pw/code/ghp6r2r2i)