235.LowestCommonAncestor.cs

// 235. 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).”
// 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.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */

public class Solution {
    public TreeNode LowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root == null)
            return root;
        
        if(root.val < p.val && root.val < q.val)
        {
            return LowestCommonAncestor(root.right , p, q);
        }
        if(root.val > p.val && root.val > q.val)
        {
            return LowestCommonAncestor(root.left , p, q);
        }
        return root;
    }
}