lowest-common-ancestor-of-a-binary-search-tree.java

// 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.
//
//
// 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.
//
//


/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        int pv = p.val, qv = q.val;
        TreeNode node = root;
        
        while (node != null) {
            int parent = node.val;
            if (pv > parent && qv > parent) {
                node = node.right;
            } else if (pv < parent && qv < parent) {
                node = node.left;
            } else {
                return node;
            }
        }
        return null;  
    }
}