recurrence for insert
insert(A, []) => [A];

insert(A, [B | X]) =>
  A < B ? [A, B | X] : [B | insert(A, X)];

Tinsert(0) = 1

Tinsert(N) < 1 + Tinsert(N-1) ;

Solving:

Tinsert(N)ë O(N).