If you consider the number 4 with the above tree
4<=5 so you traverse down the left side.
4>1 && 4>3 so you take the right child
the right child is null so 4 is not covered.
Considering 12.
12>10 so go down right branch
12<=15 so go down left branch
12>11 and 12<=12 so 12 is covered.<