[LeetCode] 235. Lowest Common Ancestor of a Binary Search Tree

Problem : https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/

Let p be smaller than q.

If root > q,  p and q's lowest common ancestor is in the left sub-tree.

If root < p,  p and q's lowest common ancestor is in the right sub-tree.

Otherwise, we find the lowest common ancestor of p and q.

Time complexity = O ( N )

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
        
        if not root:
            return root
        
        # make sure p < q
        if p.val > q.val:
            p, q = q, p
        
        if root.val > q.val:
            return self.lowestCommonAncestor(root.left, p, q)
        
        if root.val < p.val:
            return self.lowestCommonAncestor(root.right, p, q)

        return root