# Solving the recursive equation \$T(n)=T(k)+T(n-k-1)+O(n)\$

The question is clear in the title. I am trying to solve this recursion as a part of showing that the worst case of quicksort algorithm occurs when $$k=0$$, but can’t do it. I could do the following specific cases (by repeatedly using the same recursion):

1)$$T(n)=T(n-1)+O(n)$$
2)$$T(n)=2T(frac n2)+O(n)$$