LeetCode-235. Lowest Common Ancestor of a Binary Search Tree [C++][Java]_lowest common ancestor of a binary search tree c++

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题目描述

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).”

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.

 解题思路

【C++解法】

/**
 * 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 < min(p->val,q->val)) return lowestCommonAncestor(root->right, p, q);
        else if (root->val > max(p->val,q->val)) return lowestCommonAncestor(root->left, p, q);
        else return root;
    }
};

【Java解法】 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null) {return null;}
        if (root.val < Math.min(p.val, q.val)) {return lowestCommonAncestor(root.right, p, q);}
        else if (root.val > Math.max(p.val, q.val)) {return lowestCommonAncestor(root.left, p, q);}
        else {return root;}
    }
}