Data Structures, Algorithms, & Applications in C++
Chapter 11, Exercise 5

(a)

The leaves of tree (a) are abcd; those of tree (b) are abcd; and those of tree (c) are axybca

(b)

The level 3 nodes in tree (b) are + and c.

(c)

Tree (c) has 6 nodes at level 4; these are the leaves identified in part (a).