# 68. 树中两个节点的最低公共祖先
## 68.1 二叉查找树
### 题目链接
[Leetcode : 235. Lowest Common Ancestor of a Binary Search Tree](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/description/)
### 解题思路
在二叉查找树中，两个节点 p, q 的公共祖先 root 满足 root.val \>= p.val && root.val \<= q.val。
```java
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.left, p, q);
if (root.val < p.val && root.val < q.val)
return lowestCommonAncestor(root.right, p, q);
return root;
}
```
## 68.2 普通二叉树
### 题目链接
[Leetcode : 236. Lowest Common Ancestor of a Binary Tree](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/description/)
### 解题思路
在左右子树中查找是否存在 p 或者 q，如果 p 和 q 分别在两个子树中，那么就说明根节点就是最低公共祖先。
```java
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if (root == null || root == p || root == q)
return root;
TreeNode left = lowestCommonAncestor(root.left, p, q);
TreeNode right = lowestCommonAncestor(root.right, p, q);
return left == null ? right : right == null ? left : root;
}
```
