Tail
Recursion
fac(4) => f1(1,
4, 1) => f1(1, 4, 2) => f1(2,
4, 3) => f1(6, 4, 4) => f1(24,
4, 5) => 24
fac(n)
= n <= 1 ? 1 : n*fac(n-1);
f1(a, n, x) = x <= n ? 
f1(a*x, n, x+1)
: a;
f1(a*x, n, x+1) : a;tail-recursive
non-tail-recursive
fac(4) => 4*fac(3)
=> 4*3*fac(2) => 4*3*2*fac(1)=> 4*3*2*1=>4*3*2 => 4*6 => 24
fac(n)
= f1(1, n, 1);
sticky build-up