Daily LeetCode - by Vítor Belim

Description

Given the root of a binary tree, the depth of each node is the shortest distance to the root.

Return the smallest subtree such that it contains all the deepest nodes in the original tree.

A node is called the deepest if it has the largest depth possible among any node in the entire tree.

The subtree of a node is a tree consisting of that node, plus the set of all descendants of that node.

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 nodes of the tree.
Notice that nodes 5, 3 and 2 contain the deepest nodes in the tree but node 2 is the smallest subtree among them, so we return it.

Example 2:

Input: root = [1]
Output: [1]
Explanation: The root is the deepest node in the tree.

Example 3:

Input: root = [0,1,3,null,2]
Output: [2]
Explanation: The deepest node in the tree is 2, the valid subtrees are the subtrees of nodes 2, 1 and 0 but the subtree of node 2 is the smallest.

Constraints:

The number of nodes in the tree will be in the range [1, 500].
0 <= Node.val <= 500
The values of the nodes in the tree are unique.

Note: This question is the same as 1123: https://leetcode.com/problems/lowest-common-ancestor-of-deepest-leaves/

Solutions

This solution uses a single depth-first search to find the smallest subtree containing all the deepest nodes of a binary tree. The recursive dfs function walks the tree while tracking each node's depth, updating a shared height variable so it always holds the maximum depth seen so far; when it steps past a leaf (!node), it returns depth - 1, the depth of the last real node on that path. After recursing, each node learns leftDepth and rightDepth — the deepest levels reachable through its left and right subtrees — and if both equal the overall height, that node has deepest nodes on both sides (or is itself a deepest leaf), so it becomes the candidate maxNode. Because the check runs bottom-up as the recursion unwinds toward the root, the last node to satisfy it is the lowest common ancestor of all the deepest nodes, which is returned as the answer in a single O(n) pass.

Language: javascript(2026-01-09 09:59)DONE

CPU Performance100.00%

Memory Performance28.00%

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var subtreeWithAllDeepest = function (root) {
    let height = 0;
    let maxNode = null;

    const dfs = (node, depth) => {
        if (!node) return depth - 1;

        height = Math.max(height, depth);

        const leftDepth = dfs(node.left, depth + 1);
        const rightDepth = dfs(node.right, depth + 1);

        if (leftDepth === height && rightDepth === height) {
            maxNode = node;
        }

        return Math.max(leftDepth, rightDepth);
    };

    dfs(root, 0);

    return maxNode;
};

Description

Solutions

AI Explanation