Given the root of a binary search tree and an integer k, determine whether there are two elements in the BST whose sum equals k. Return TRUE if such elements exist or FALSE otherwise.

Constraints:

• The number of nodes in the tree is in the range $[1, 10^3]$.

• $-10^3 \leq$ Node.data $\leq 10^3$

• root is guaranteed to be a valid binary search tree.

• $-10^4 \leq$ k $\leq 10^4$