題意大概是，找兩個節點的最低公共祖先

/**
 * 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) {
        
 
if (root == NULL)
            return NULL;
        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;   //出現了一大一小，就可以返回root了
    }
};

【easy】235. Lowest Common Ancestor of a Binary Search Tree