235. Lowest Common Ancestor of a Binary Search Tree
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 p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”
Given binary search tree: root = [6,2,8,0,4,7,9,null,null,3,5]
Example 1:
Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8
Output: 6
Explanation: The LCA of nodes 2 and 8 is 6
Example 2:
Input: root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4
Output: 2
Explanation: The LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.
Note:
- All of the nodes' values will be unique.
- p and q are different and both values will exist in the BST.
题目链接：https://leetcode-cn.com/problems/lowest-common-ancestor-of-a-binary-search-tree/
思路
由于是二叉搜索树，由二叉搜索树的性质辅助判断。
法一：递归
/**
* 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 || !p || !q) return NULL;
if(p->val==root->val || q->val==root->val
|| ((p->val < root->val)!=(q->val < root->val))) return root;
else{
if(p->val<root->val) return lowestCommonAncestor(root->left, p, q);
else return lowestCommonAncestor(root->right, p, q);
}
}
};法二：非递归
/**
* 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 || !p || !q) return NULL;
while(root){
if(p->val==root->val || q->val==root->val
|| ((p->val < root->val)!=(q->val < root->val))) return root;
if(p->val<root->val) root = root->left;
else root = root->right;
}
return NULL;
}
};