0235_lowest_common_ancestor_of_bst.cpp

/*
* Copyright(c) 2019 Jiau Zhang
* For more information see <https://github.com/JiauZhang/algorithms>
* 
* This repo is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation
*
* It is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with THIS repo. If not, see <http://www.gnu.org/licenses/>.
*/

/*
* https://leetcode-cn.com/problems/lowest-common-ancestor-of-a-binary-search-tree
* 题目描述：
*     给定一个二叉搜索树, 找到该树中两个指定节点的最近公共祖先
*     
* 解题思路：
*     要么左子树，要么右子树，否则当前节点就是最低公共祖先
*/

/**
 * 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 == nullptr)
            return root;

        if (p->val > root->val && q->val > root->val)
            return lowestCommonAncestor(root->right, p, q);
        else if (p->val < root->val && q->val < root->val)
            return lowestCommonAncestor(root->left, p, q);
        else
            return root;
    }
};