Use fast and slow (or tortoise) pointers in each route of the tree.

Advance fast pointer towards leaf by 2.
Advance slow pointer towards lead by 1.
If fast pointer reaches the leaf print value at the slow pointer
Check if the fast->left->left exists, then recursively move slow pointer by one step and fast pointer by two steps.
If the fast->left->left doesn’t exist (in case of even number of levels), the move both the pointers by one step.

Print middle level of perfect binary tree without finding height  : https://www.geeksforgeeks.org/print-middle-level-perfect-binary-tree-without-finding-height/