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

class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q)
    {
        if (root == NULL || p == NULL || q == NULL)
            return NULL;
        while ((root != NULL) && (root != p) && (root != q) && ((root->val > p->val && root->val > q->val) || (root->val < p->val && root->val < q->val)))
        {
            if (root->val > p->val)
                root = root->left;
            else
                root = root->right;
        }
        return root;
    }
};