# 236. Lowest Common Ancestor of a Binary Tree

[Link](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/)

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

According to the [definition of LCA on Wikipedia](https://en.wikipedia.org/wiki/Lowest_common_ancestor): “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 the following binary tree:  root = \[3,5,1,6,2,0,8,null,null,7,4]![](https://assets.leetcode.com/uploads/2018/12/14/binarytree.png)

**Example 1:**

```
Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
Output: 3
Explanation: The LCA of nodes 5 and 1 is 3.
```

**Example 2:**

```
Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
Output: 5
Explanation: The LCA of nodes 5 and 4 is 5, 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 binary tree.

**Solution**

這題很巧妙的使用lowestCommonAncestor function的特性

當 Tree 不包含 p, q時，回傳會是null

當Tree只含其中之一，那回傳會是p or q

當Tree包含p, q, 那回傳的就是會是lowestCommonAncestor

```
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
       if(root == null || root = q || root == p) return root;
       TreeNode left = lowestCommonAncestor(root.left, p ,q);
       TreeNode right = lowestCommonAncestor(root.right, p ,q);
       if(left != null && left != null){
          return root;
       }
       return left == null ? right : left;
    }
}
```
