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1790-lowest-common-ancestor-of-a-binary-tree-iii/README.md

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+<p>Given two nodes of a&nbsp;binary tree <code>p</code> and <code>q</code>, return <em>their&nbsp;lowest common ancestor (LCA)</em>.</p>
+
+<p>Each node will have a reference to its parent node. The definition for <code>Node</code> is below:</p>
+
+<pre>
+class Node {
+    public int val;
+    public Node left;
+    public Node right;
+    public Node parent;
+}
+</pre>
+
+<p>According to 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 two nodes p and q in a tree T is the lowest node that has both p and q as descendants (where we allow <b>a node to be a descendant of itself</b>).&quot;</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" style="width: 200px; height: 190px;" />
+<pre>
+<strong>Input:</strong> root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
+<strong>Output:</strong> 3
+<strong>Explanation:</strong> The LCA of nodes 5 and 1 is 3.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2018/12/14/binarytree.png" style="width: 200px; height: 190px;" />
+<pre>
+<strong>Input:</strong> root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
+<strong>Output:</strong> 5
+<strong>Explanation:</strong> The LCA of nodes 5 and 4 is 5 since a node can be a descendant of itself according to the LCA definition.
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> root = [1,2], p = 1, q = 2
+<strong>Output:</strong> 1
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li>The number of nodes in the tree is in the range <code>[2, 10<sup>5</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><code>p != q</code></li>
+	<li><code>p</code> and <code>q</code> exist in the tree.</li>
+</ul>

1790-lowest-common-ancestor-of-a-binary-tree-iii/solution.py

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+# Time: O(h), h = height of the tree
+# Space: O(h)
+
+"""
+# Definition for a Node.
+class Node:
+    def __init__(self, val):
+        self.val = val
+        self.left = None
+        self.right = None
+        self.parent = None
+"""
+
+class Solution:
+    def lowestCommonAncestor(self, p: 'Node', q: 'Node') -> 'Node':
+        visited = set()
+
+        while q:
+            visited.add(q.val)
+            q = q.parent
+
+        while p:
+            if p.val in visited:
+                return p
+            visited.add(p.val)
+            p = p.parent
+
+        return None
+