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).”


  • 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


The value of the LCA between nodes p and q


Example 1:


1bst = [6,2,8,0,4,7,9,x,x,3,5]
2p = 2
3q = 8

Output: 6


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.

Try it yourself




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1  >>> a = [1, 2, 3]
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3  3

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