why Level does not sort output according to their levels

Question

Consider this example:

Level[a + f[x, y^n], {0, Infinity}]

(*{a, x, y, n, y^n, f[x, y^n], a + f[x, y^n]}*)

Why the output is sorted in this way. why not following their levels? I was expecting to get:

(*{a + f[x, y^n], a, f[x, y^n], x, y^n, y, n}*)

Answer 1

As Kuba comments Level follows the standard expression traversal order:

Level traverses expressions in depth-first order, so that the subexpressions in the final list are ordered lexicographically by their indices.

This is actually a depth-first postorder traversal.
It is the normal order in which expressions are evaluated in Mathematica:

echo[x_] := (Print@x; x)

expr = a + f[x, y^n];

Map[echo, HoldForm @@ {expr}, {1, -1}]

ReleaseHold[%];

echo[echo[a] + echo[f[echo[x], echo[echo[y]^echo[n]]]]]

a
x
y
n
y^n
f[x,y^n]
a+f[x,y^n]

It is used by nearly all System traversal functions, e.g.:

Scan[Print, expr, {0, -1}]  (* same output as above; omitted *)

Cases[expr, _, {0, -1}]

expr ~Extract~ Position[expr, _, {0, -1}, Heads -> False]

{a, x, y, n, y^n, f[x, y^n], a + f[x, y^n]}
{a, x, y, n, y^n, f[x, y^n], a + f[x, y^n]}

A notable exception is ReplaceAll which uses a depth-first preorder traversal. See:

• How to perform a depth-first preorder traversal of an expression?

You are interested in a breadth-first traversal; see:

• How to perform a breadth-first traversal of an expression?

For an understanding of levels as used by Mathematica see the excellent illustrations in answer to:

• Levels: how do they work?

  Don't miss Rahul Narain's answer; while less popular than David's it is concise and accurate.

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Solution to implied problem

Although I hope the information above including the linked Q&A's will give you the knowledge to solve the implied problem yourself here is my take on Kuba's code:

expr = a + f[x, y^n];

Array[expr ~Level~ {#} &, Depth @ expr, 0, Join]

{a + f[x, y^n], a, f[x, y^n], x, y^n, y, n}

And a solution using WReach's bf function from the link above:

Reap[Sow ~bf~ expr][[2, 1]]

{a + f[x, y^n], a, f[x, y^n], x, y^n, y, n}