# LeetCode-Alg-235-Lowest Common Ancestor of a Binary Search Tree

## LeetCode-Alg-235-Lowest Common Ancestor of a Binary Search Tree

Posted by Jae on May 5, 2019

### 1、题目

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).”

Given binary search tree: root = [6,2,8,0,4,7,9,null,null,3,5]

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.


Note:

All of the nodes’ values will be unique. p and q are different and both values will exist in the BST.

### 4、实现

public class Solution {

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if (p.val > q.val){
return lowestCommonAncestor(root, q, p);
}

if (root == null){
return null;
}

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