Leetcode 235. Lowest Common Ancestor of a Binary Search Tree

Posted 2020-07-31 Deribs4

235. Lowest Common Ancestor of a Binary Search Tree

Total Accepted: 77021 Total Submissions: 205265 Difficulty: Easy

 

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

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

_______6______
       /                  ___2__          ___8__
   /      \        /         0      _4       7       9
         /           3   5

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

 

思路：

二叉排序树(Binary Sort Tree)又称二叉查找(搜索)树(Binary Search Tree)。其定义为：二叉排序树或者是空树，或者是满足如下性质的二叉树：

①若它的左子树非空，则左子树上所有结点的值均小于根结点的值；

②若它的右子树非空，则右子树上所有结点的值均大于根结点的值；

③左、右子树本身又各是一棵二叉排序树。

 

代码：
迭代：

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
13         while(true){
14             if(root->val>p->val&&root->val>q->val){
15                 root=root->left;
16                 continue;
17             }
18             if(root->val<p->val&&root->val<q->val){
19                 root=root->right;
20                 continue;
21             }
22             return root;
23         }
24     }
25 };

 

递归：

1 class Solution {
2 public:
3     TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
4         if(root->val>p->val&&root->val>q->val) return lowestCommonAncestor(root->left,p,q);
5         if(root->val<p->val&&root->val<q->val) return lowestCommonAncestor(root->right,p,q);
6         return root;
7     }
8 };

 

还有另外1题，和这题相似：Leetcode 236. Lowest Common Ancestor of a Binary Tree