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1816-lowest-common-ancestor-of-a-binary-tree-iv/README.md

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+<p>Given the <code>root</code> of a binary tree and an array of <code>TreeNode</code> objects <code>nodes</code>, return <em>the lowest common ancestor (LCA) of <strong>all the nodes</strong> in </em><code>nodes</code>. All the nodes will exist in the tree, and all values of the tree&#39;s nodes are <strong>unique</strong>.</p>
+
+<p>Extending the <strong><a href="https://en.wikipedia.org/wiki/Lowest_common_ancestor" target="_blank">definition of LCA on Wikipedia</a></strong>: &quot;The lowest common ancestor of <code>n</code> nodes <code>p<sub>1</sub></code>, <code>p<sub>2</sub></code>, ..., <code>p<sub>n</sub></code> in a binary tree <code>T</code> is the lowest node that has every <code>p<sub>i</sub></code> as a <strong>descendant</strong> (where we allow <b>a node to be a descendant of itself</b>) for every valid <code>i</code>&quot;. A <strong>descendant</strong> of a node <code>x</code> is a node <code>y</code> that is on the path from node <code>x</code> to some leaf node.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2018/12/14/binarytree.png" />
+<pre>
+<strong>Input:</strong> root = [3,5,1,6,2,0,8,null,null,7,4], nodes = [4,7]
+<strong>Output:</strong> 2
+<strong>Explanation:</strong> The lowest common ancestor of nodes 4 and 7 is node 2.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2018/12/14/binarytree.png" />
+<pre>
+<strong>Input:</strong> root = [3,5,1,6,2,0,8,null,null,7,4], nodes = [1]
+<strong>Output:</strong> 1
+<strong>Explanation:</strong> The lowest common ancestor of a single node is the node itself.
+
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2018/12/14/binarytree.png" />
+<pre>
+<strong>Input:</strong> root = [3,5,1,6,2,0,8,null,null,7,4], nodes = [7,6,2,4]
+<strong>Output:</strong> 5
+<strong>Explanation:</strong> The lowest common ancestor of the nodes 7, 6, 2, and 4 is node 5.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li>The number of nodes in the tree is in the range <code>[1, 10<sup>4</sup>]</code>.</li>
+	<li><code>-10<sup>9</sup> &lt;= Node.val &lt;= 10<sup>9</sup></code></li>
+	<li>All <code>Node.val</code> are <strong>unique</strong>.</li>
+	<li>All <code>nodes[i]</code> will exist in the tree.</li>
+	<li>All <code>nodes[i]</code> are distinct.</li>
+</ul>

1816-lowest-common-ancestor-of-a-binary-tree-iv/solution.py

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+# Time: O(n)
+# Space: O(n + m), O(n) for the recursive call stack, O(m) for the set `seen`
+
+# 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', nodes: 'List[TreeNode]') -> 'TreeNode':
+        seen = set(nodes)
+
+        def dfs(root):
+            if root in seen:
+                return root
+
+            if not root:
+                return None
+
+            left = dfs(root.left)
+            right = dfs(root.right)
+
+            if left and right:
+                return root
+
+            if left and not right:
+                return left
+
+            if right and not left:
+                return right
+
+            return None
+
+        return dfs(root)
+