Trees & BST

A binary search tree $T$ contains $n$ distinct elements. What is the time complexity of picking an element in $T$ that is smaller than the maximum element in $T$?

Consider a complete binary tree with 7 nodes. Let $A$ denote the set of first 3 elements obtained by performing Breadth-First Search (BFS) starting from the root. Let $B$ denote the set of first 3 elements obtained by performing Depth-First Search (DFS) starting from the root. The value of $|A - B|$ is __________.