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;
}
}```

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