if a ≤ 0
return a;
but note that returned value doesn't use anywhere ====> just ignore the values if a ≤ 0
note that 'a' is a auto variable.
Rec(6) = print(6) Rec(4) print(6) Rec(3) ----- (1)
Rec(4) = print(4) Rec(2) print(4) Rec(1)
Rec(2) = print(2) Rec(0) print(2) Rec(-1) ===> 2 2
Rec(1) = print(1) Rec(-1) print(1) Rec(-2) ===> 1 1
∴ Rec(4) = 4 2 2 4 1 1
Rec(3) = print(3) Rec(1) print(3) Rec(0)
Rec(1) = print(1) Rec(-1) print(1) Rec(-2) ===> 1 1
∴ Rec(3) = 3 1 1 3
∴ Rec(6) = 6 4 2 2 4 1 1 6 3 1 1 3