# 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.

``````class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
if (!root) return NULL;
if (root->val > max(p->val, q->val))
return lowestCommonAncestor(root->left, p, q);
else if (root->val < min(p->val, q->val))
return lowestCommonAncestor(root->right, p, q);
else return root;
}
};
``````

``````class Solution {
public:
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
while (true) {
if (root->val > max(p->val, q->val)) root = root->left;
else if (root->val < min(p->val, q->val)) root = root->right;
else break;
}
return root;
}
};
``````

Github 同步地址：

https://github.com/grandyang/leetcode/issues/235

Lowest Common Ancestor of a Binary Tree

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

https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/discuss/64980/C%2B%2B-Recursive-and-Iterative

https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/discuss/64963/3-lines-with-O(1)-space-1-Liners-Alternatives

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