LeetCode 235. Lowest Common Ancestor of a Binary Search Tree

235. Lowest Common Ancestor of a Binary Search Tree

My Submissions QuestionEditorial Solution

Total Accepted: 63643 Total Submissions: 168231 Difficulty: Easy

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 v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

_______6______

/ \

___2__ ___8__

/ \ / \

0 _4 7 9

/ \

3 5

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

/**
* 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 || p==NULL || q==NULL)
return NULL;
if((p->val >= root->val && q->val <= root ->val) || (p->val <= root->val && q->val >= root->val)) {
return root;
}
if(p->val > root->val && q->val > root->val) {
return lowestCommonAncestor(root->right, p, q);
}
if(p->val < root->val && q->val < root->val) {
return lowestCommonAncestor(root->left, p, q);
}
}
};