You have to make binary search tree of keys 10, 9, 8, 7, 6, 5, 4, 3, 2,1
It will be O($n^2$) since skew bst will be work as insersion sort .
For 1st element no comparision
For 2nd element 1 comparision
For 3rd element 2 comparision
.......
total comparision : 1+2+3+4+5+6...n-1 = O($n^2$)