update README.md 21.2.md

added unsolved: 21.2.5

C21-Data-Structures-for-Disjoint-Sets/21.2.md

@@ -54,14 +54,12 @@ T = O(n) + O(n) = O(n)
 
 This example is the best case, because in each UNION, we only move one element.
 
-### Exercises 21.2-5
+### Exercises 21.2-5 
 ***
 Suggest a simple change to the UNION procedure for the linked-list representation that removes the need to keep the tail pointer to the last object in each list. Whether or not the weighted-union heuristic is used, your change should not change the asymptotic running time of the UNION procedure. (Hint: Rather than appending one list to another, splice them together.)
 
 ### `Answer`
-Actually, I don't the meaning of doing such modifications...
-
-If you have ideas, please merge it to me.
+UNSOLVED
 
 ***
 Follow [@louis1992](https://github.com/gzc) on github to help finish this task.