Problem #236: Lowest Common Ancestor of a Binary Tree

Intuition & Strategy Breakdown

Recursively search left and right subtrees. If root matches p or q, or if both subtrees return non-null nodes, root is the LCA.

Python Solution

class Solution:
    def lowestCommonAncestor(self, root: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
        if not root or root == p or root == q:
            return root
        
        left = self.lowestCommonAncestor(root.left, p, q)
        right = self.lowestCommonAncestor(root.right, p, q)
        
        if left and right:
            return root
        
        return left or right

Java Solution

class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if (root == null || root == p || root == q) {
            return root;
        }
        
        TreeNode left = lowestCommonAncestor(root.left, p, q);
        TreeNode right = lowestCommonAncestor(root.right, p, q);
        
        if (left != null && right != null) {
            return root;
        }
        
        return left != null ? left : right;
    }
}