Day 20.1.txt

Lowest Common Ancestor of a Binary Search Tree

Given a binary search tree (BST), find the lowest common ancestor (LCA) 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
Explanation: The LCA of nodes 2 and 8 is 6.
Example 2:


Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4
Output: 2
Explanation: The LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.
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.



class Solution {
    public TreeNode lowestCommonAncestor(TreeNode r, TreeNode p, TreeNode q) {
        if(r.val>p.val&&r.val>q.val)
            return lowestCommonAncestor(r.left,p,q);
        else if(r.val<p.val&&r.val<q.val)
            return lowestCommonAncestor(r.right,p,q);
        else
            return r;
    }
}