Is Lowest Common Ancestor of Deepest Leaves easy or hard?

Lowest Common Ancestor of Deepest Leaves is rated Medium difficulty. The challenge is recognizing that the deepest leaves determine the ancestor based on subtree depth comparison. Once you frame the problem as a DFS returning depth and node together, the implementation becomes straightforward.

Lowest Common Ancestor of Deepest Leaves Python/Java solution

Python and Java implementations usually follow the same DFS pattern: recursively compute left and right subtree depths, then return the node that represents the LCA when both depths match. The logic fits naturally with recursive functions in Python and helper methods in Java classes.

How to solve Lowest Common Ancestor of Deepest Leaves in O(n)?

Perform a post-order DFS where each recursive call returns the maximum depth and the LCA candidate for that subtree. Compare the left and right subtree depths. If they are equal, return the current node as the LCA; otherwise return the deeper subtree's candidate. Since every node is processed once, the complexity is O(n).

What is the best approach for Lowest Common Ancestor of Deepest Leaves?

The most efficient approach uses a single Depth-First Search traversal that returns both the deepest depth and the corresponding LCA node from each subtree. If the left and right subtree depths are equal, the current node becomes the LCA. This method runs in O(n) time and O(h) space where h is the height of the tree.

Is Lowest Common Ancestor of Deepest Leaves asked at Google/Amazon/Meta?

Tree and LCA variations frequently appear in interviews at companies like Google, Amazon, and Meta. This problem tests understanding of binary tree traversal, depth tracking, and combining subtree results, which are common interview themes.

What data structure is used in Lowest Common Ancestor of Deepest Leaves?

The primary data structure is a binary tree. The solution typically uses Depth-First Search recursion or Breadth-First Search with a queue. Some implementations also maintain parent pointers or hash maps when computing the LCA of multiple nodes.

What is the time complexity of Lowest Common Ancestor of Deepest Leaves?

The optimal solution runs in O(n) time because each node in the binary tree is visited exactly once during traversal. Recursive DFS uses O(h) space for the call stack, while a BFS-based solution may require O(n) space for the queue and parent tracking.

Description Hints 3 Solution Video 42:25

1123. Lowest Common Ancestor of Deepest Leaves

Medium79.5% acceptance4 companies

Given the root of a binary tree, return the lowest common ancestor of its deepest leaves.

Recall that:

The node of a binary tree is a leaf if and only if it has no children
The depth of the root of the tree is 0. if the depth of a node is d, the depth of each of its children is d + 1.
The lowest common ancestor of a set S of nodes, is the node A with the largest depth such that every node in S is in the subtree with root A.

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4]
Output: [2,7,4]
Explanation: We return the node with value 2, colored in yellow in the diagram.
The nodes coloured in blue are the deepest leaf-nodes of the tree.
Note that nodes 6, 0, and 8 are also leaf nodes, but the depth of them is 2, but the depth of nodes 7 and 4 is 3.

Example 2:

Input: root = [1]
Output: [1]
Explanation: The root is the deepest node in the tree, and it's the lca of itself.

Example 3:

Input: root = [0,1,3,null,2]
Output: [2]
Explanation: The deepest leaf node in the tree is 2, the lca of one node is itself.

Constraints:

The number of nodes in the tree will be in the range [1, 1000].
0 <= Node.val <= 1000
The values of the nodes in the tree are unique.

Note: This question is the same as 865: https://leetcode.com/problems/smallest-subtree-with-all-the-deepest-nodes/

Asked at 4 companies

Topics

Hints open one at a time

Hint 1

Do a postorder traversal.

Hint 2

Then, if both subtrees contain a deepest leaf, you can mark this node as the answer (so far).

Hint 3

The final node marked will be the correct answer.

42:25

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