L235. Lowest Common Ancestor of a Binary Search Tree

recursion

Problem:

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.

Solution:

• According to the binary search tree definition, left node always < root < right node;

• so if p > q, change the order of them

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    if(root == null || p == null || q == null) return null;
    if(root == p || root == q) return root;
    if(p == q) return p;
    if(p.val > q.val) return lowestCommonAncestor(root, q, p);
    if(root.val > p.val && root.val < q.val) return root;
    else if(root.val < p.val) return lowestCommonAncestor(root.right, p, q);
    else return lowestCommonAncestor(root.left, p, q);
}