235. Lowest Common Ancestor of a Binary Search Tree

Description

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).”

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.

Example 3:

Input: root = [2,1], p = 2, q = 1
Output: 2

Constraints:

• The number of nodes in the tree is in the range [2, 105].
• 109 <= Node.val <= 109
• All Node.val are unique.
• p != q
• p and q will exist in the BST.

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Solutions

Solution 1

Intuition

it is a binary search tree, you only need find a node with mid value

Code

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    int m = p.val;
    int n = q.val;
    while (true) {
        if (root.val > m && root.val > n) {
            root = root.left;
        } else if (root.val < m && root.val < n) {
            root = root.right;
        } else {
            break;
        }
    }
    return root;

}

Complexity

• Time: O(logn)
• Space: O(h)

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Solution 2

Intuition

Code

Complexity

• Time:
• Space:

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