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 ________.

A complete binary tree with 7 nodes has 3 levels:

• Level 0: root node, say $1$

• Level 1: left child $2$, right child $3$

• Level 2: children of $2$ are $4, 5$; children of $3$ are $6, 7$

1. Breadth-First Search (BFS) visits nodes level by level:

   • Starting at the root, the first 3 nodes visited are the root and its two direct children (Level 0 and Level 1).

   • Thus, $A = \{1, 2, 3\}$.

2. Depth-First Search (DFS) explores as deep as possible along each branch before backtracking:

   • Starting at the root ($1$), it visits a child (e.g., $2$) and then a grandchild (e.g., $4$).

   • Thus, the first 3 nodes visited are $B = \{1, 2, 4\}$ (or $\{1, 3, 6\}$ if visiting right child first).

3. Set Difference:

   $$A - B = \{1, 2, 3\} - \{1, 2, 4\} = \{3\}$$

   $$|A - B| = 1$$

Therefore, the value of $|A - B|$ is 1.