0235-lowest-common-ancestor-of-a-binary-search-tree.cpp

/*
    Given a binary search tree (BST), find the LCA of 2 given nodes in the BST

    Use BST property: if curr > left & right go left, else if < go right, else done

    Time: O(n)
    Space: O(n)
*/

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

// class Solution {
// public:
//     TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
//         while (root != NULL) {
//             if (p->val < root->val && q->val < root->val) {
//                 root = root->left;
//             } else if (p->val > root->val && q->val > root->val) {
//                 root = root->right;
//             } else {
//                 return root;
//             }
//         }
//         return NULL;
//     }
// };