235. 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 v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

        _______6______
       /              \
    ___2__          ___8__
   /      \        /      \
   0      _4       7       9
         /  \
         3   5

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is 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 {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {     
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {  
        if(root==null || p==null || q==null) return null;  

        if(Math.max(p.val, q.val) < root.val) {  
            return lowestCommonAncestor(root.left, p, q);  
        } else if(Math.min(p.val, q.val) > root.val) {  
            return lowestCommonAncestor(root.right, p, q);  
        } else return root;  
    }  
}