Lowest Common Ancestor of a Binary Search Tree

Given a binary search tree (BST), find the lowest common ancestor (LCA) node of two given nodes in the BST.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

Input

• bst: a binary tree representing the existing BST.
• p: the value of node p as described in the question
• q: the value of node q as described in the question

Output

The value of the LCA between nodes p and q

Examples

Example 1:

Input:

bst = [6,2,8,0,4,7,9,x,x,3,5]
p = 2
q = 8

Output: 6

Explanation:

The ancestors of node p with value 2 are node 2 and node 6. The ancestors of node q with value 8 are node 8 and node 6.

The lowest common ancestors between these two nodes is 6.

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Solution