235. Lowest Common Ancestor of a Binary Search Tree

Recursion

/**
 * 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);
        }

        if (p->val > root->val && q->val > root->val)
        {
            return lowestCommonAncestor(root->right, p, q);
        }
        return root;
    }
};

• T: $O(N)$
• S: $O(N)$

Iteration

/**
 * 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)
    {
        TreeNode* node = root;
        while (true)
        {
            int parentVal = node->val;
            if (parentVal < p->val && parentVal < q->val)
            {
                node = node->right;
            }
            else if (parentVal > p->val && parentVal > q->val)
            {
                node = node->left;
            }
            else
            {
                return node;
            }
        }
        return node;
    }
};

• T: $O(N)$
• S: $O(1)$