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) return null;
        if(root.val == p.val || root.val == q.val) return root;
        if(root.val > p.val && root.val > q.val){
            return lowestCommonAncestor(root.left, p,q);
        }else if(root.val < p.val && root.val < q.val){
            return lowestCommonAncestor(root.right, p,q);
        }else{
            return root;
        }
    }
}

public class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root == null) return null;

        if((root.val >= p.val && root.val <= q.val)
                || (root.val <= p.val && root.val >= q.val)) return root;
        else{
          if(root.val < p.val && root.val < q.val) root = root.right;
          else root = root.left;

          return lowestCommonAncestor(root, p, q);
        }  
    }
}