Added explanation on Queue computed size.

Author: aubertc

source/lectures/data/queue.md

@@ -104,3 +104,12 @@ public int CSize
 ```
 
 but the exception would be thrown: can you figure out in which case(s) the "computed size" would be incorrect?
+<details>
+<summary>Solution</summary>
+The implementation is *almost* correct, the problem is that `front == end` can be true for two reasons:
+
+#. The queue is empty, no element have been added yet (hence, size should be 0),
+#. The queue is full, no element can be added (hence, size should be `mArray.Length`).
+
+Since `IsFull`, `IsEmpty` and `Capacity` all uses `size`, we have no way of telling if we are in the first or second situation. The rest of the implementation is correct, but `size` must be an attribute to be able to differenciate those two cases (note that we could also have added a `bool` to distinguish between "full" and "empty", but it's more convenient to have a `size` attribute available at all time).
+</details>