0865. Smallest Subtree with all the Deepest Nodes
https://leetcode.com/problems/smallest-subtree-with-all-the-deepest-nodes
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 tree consisting of that node, plus the set of all descendants of that node.
Note: This question is the same as 1123: https://leetcode.com/problems/lowest-common-ancestor-of-deepest-leaves/
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.
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