235. Lowest Common Ancestor of a Binary Search Tree

Medium (Trung bình) C++ 🔗 Xem trên LeetCode

📋 Đề Bài

Given a binary search tree (BST), find the lowest common ancestor (LCA) node 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.

Example 3:

Input: root = [2,1], p = 2, q = 1
Output: 2

 

Constraints:

  • The number of nodes in the tree is in the range [2, 105].
  • -109 <= Node.val <= 109
  • All Node.val are unique.
  • p != q
  • p and q will exist in the BST.

🧠 Thuật Toán & Kỹ Thuật

Tree Traversal (Duyệt cây)
⏱️ Thời gian O(n)
💾 Không gian O(1)

💻 Lời Giải

C++ 0235-lowest-common-ancestor-of-a-binary-search-tree.cpp 
/**
 * 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->val > p->val and root->val > q->val) {
            return lowestCommonAncestor(root->left, p, q);
        }
        else if (root->val < p->val and root->val < q->val) {
            return lowestCommonAncestor(root->right, p, q);
        }
        return root;
    }
};