How many distinct binary search trees can be formed using 5 distinct keys?

Options

  • A. 14
  • B. 42
  • C. 120
  • D. More than one of the above
  • E. None of the above

Correct Answer (Detailed Explanation is Below)

B. 42

Detailed Explanation

The number of distinct BSTs formed from N distinct keys is the Catalan number CN. For five keys, C5 = 42. The recurrence can also be written as CN = Σ CiCN-1-i, where each possible key is considered as the root and the remaining keys are divided between the left and right subtrees. This recurrence explains why the number grows rapidly.