---
# System prepended metadata

title: 'Leetcode [No. 235] Lowest Common Ancestor of a Binary Search Tree(MEDIUM)'
tags: [leetcode]

---

# Leetcode [No. 235] Lowest Common Ancestor of a Binary Search Tree(MEDIUM)

## 題目

https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/description/

## 思路
+ 這個解法運用了BinarySearchTree的特性
```c++
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */

class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        TreeNode* cur = root;
        int biggerVal = max(p->val, q->val);
        int smallerVal = min(p->val, q->val);
        while(cur!=nullptr)
        {
            if(cur->val > biggerVal)
            {
                cur = cur->left; // this 
            }
            else if (cur->val < smallerVal)
            {
                cur = cur->right;
            }
            else // in the corrent bound
            {
                return cur;
            }
        }
        return nullptr;
    }
};
```

### 解法分析
+ time complexity: O(lgn)


### 執行結果

![image](https://hackmd.io/_uploads/BJy2PUnU6.png)
