Leetcode

235. Lowest Common Ancestor of a Binary Search Tree

by hooni posted May 01, 2020
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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.


/**
 * 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) {
        TreeNode pNode = p;
        TreeNode qNode = q;
        TreeNode node = root;
        
        while(node != null){
            TreeNode parent = node;
            
            if(parent.val < qNode.val && parent.val < pNode.val){
                node = node.right;
            }else if(qNode.val < parent.val && pNode.val < parent.val){
                node = node.left;
            }else{
                return node;
            }
        }
        return null;
    }
}


/**
 * 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) {
        TreeNode pNode = p;
        TreeNode qNode = q;
        
        if(root.val < qNode.val && root.val < pNode.val){
            return lowestCommonAncestor(root.right, p, q);
        }else if(qNode.val < root.val && pNode.val < root.val){
            return lowestCommonAncestor(root.left, p, q);
        }
        return root;
    }
}

[문제] https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/