add leetcode problem-cn part2

算法题（国内版）/problem (English)/首个共同祖先(English) [first-common-ancestor-lcci].html

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+<p>Design an algorithm and write code to find the first common ancestor of two nodes in a binary tree. Avoid storing additional nodes in a data structure. NOTE: This is not necessarily a binary search tree.</p>
+
+<p>For example, Given the following tree: root = [3,5,1,6,2,0,8,null,null,7,4]</p>
+
+<pre>
+    3
+   / \
+  5   1
+ / \ / \
+6  2 0  8
+  / \
+ 7   4
+</pre>
+
+<p><strong>Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
+<strong>Input:</strong> 3
+<strong>Explanation:</strong> The first common ancestor of node 5 and node 1 is node 3.</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<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 first common ancestor of node 5 and node 4 is node 5.</pre>
+
+<p><strong>Notes:</strong></p>
+
+<ul>
+	<li>All node values are pairwise distinct.</li>
+	<li>p, q are different node and both can be found in the given tree.</li>
+</ul>