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1218-lowest-common-ancestor-of-deepest-leaves/README.md

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+<p>Given the <code>root</code> of a binary tree, return <em>the lowest common ancestor of its deepest leaves</em>.</p>
+
+<p>Recall that:</p>
+
+<ul>
+	<li>The node of a binary tree is a leaf if and only if it has no children</li>
+	<li>The depth of the root of the tree is <code>0</code>. if the depth of a node is <code>d</code>, the depth of each of its children is <code>d + 1</code>.</li>
+	<li>The lowest common ancestor of a set <code>S</code> of nodes, is the node <code>A</code> with the largest depth such that every node in <code>S</code> is in the subtree with root <code>A</code>.</li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://s3-lc-upload.s3.amazonaws.com/uploads/2018/07/01/sketch1.png" style="width: 600px; height: 510px;" />
+<pre>
+<strong>Input:</strong> root = [3,5,1,6,2,0,8,null,null,7,4]
+<strong>Output:</strong> [2,7,4]
+<strong>Explanation:</strong> 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.</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> root = [1]
+<strong>Output:</strong> [1]
+<strong>Explanation:</strong> The root is the deepest node in the tree, and it&#39;s the lca of itself.
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> root = [0,1,3,null,2]
+<strong>Output:</strong> [2]
+<strong>Explanation:</strong> The deepest leaf node in the tree is 2, the lca of one node is itself.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li>The number of nodes in the tree will be in the range <code>[1, 1000]</code>.</li>
+	<li><code>0 &lt;= Node.val &lt;= 1000</code></li>
+	<li>The values of the nodes in the tree are <strong>unique</strong>.</li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong>Note:</strong> This question is the same as 865: <a href="https://leetcode.com/problems/smallest-subtree-with-all-the-deepest-nodes/" target="_blank">https://leetcode.com/problems/smallest-subtree-with-all-the-deepest-nodes/</a></p>

1218-lowest-common-ancestor-of-deepest-leaves/solution.py

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+# Approach 1: Recursion
+
+# Time: O(n)
+# Space: O(n)
+
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+
+class Solution:
+    def lcaDeepestLeaves(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
+        def dfs(root):
+            if not root:
+                return 0, None
+
+            left = dfs(root.left)
+            right = dfs(root.right)
+
+            if left[0] > right[0]:
+                return left[0] + 1, left[1]
+            if left[0] < right[0]:
+                return right[0] + 1, right[1]
+            return left[0] + 1, root
+
+        return dfs(root)[1]
+