Lowest Common Ancestor of a Binary Search Tree
Medium
Tree
Depth-First Search
Binary Search Tree
Binary 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)."
Example 1
Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8
Output: 6
Example 2
Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4
Output: 2
Example 3
Input: root = [2,1], p = 2, q = 1
Output: 2

Constraints

  • •The number of nodes in the tree is in the range [2, 10⁵].
  • •-10⁹ <= Node.val <= 10⁹
  • •All Node.val are unique.
  • •p != q
  • •p and q will exist in the BST.
Time Complexity
O(h)
Space Complexity
O(1)
14
Case 1
Input: [6,2,8,0,4,7,9,null,null,3,5] 2 8
Expected: 6
Case 2
Input: [6,2,8,0,4,7,9,null,null,3,5] 2 4
Expected: 2
Case 3
Input: [2,1] 2 1
Expected: 2