add leetcode problem-cn part2

算法题（国内版）/problem (English)/检查平衡性(English) [check-balance-lcci].html

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+<p>Implement a function to check if a binary tree is balanced. For the purposes of this question, a balanced tree is defined to be a tree such that the heights of the two subtrees of any node never differ by more than one.</p>
+
+<p><br />
+<strong>Example 1:</strong></p>
+
+<pre>
+Given tree [3,9,20,null,null,15,7]
+    3
+   / \
+  9  20
+    /  \
+   15   7
+return true.</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<pre>
+Given [1,2,2,3,3,null,null,4,4]
+      1
+     / \
+    2   2
+   / \
+  3   3
+ / \
+4   4
+return&nbsp;false.</pre>
+
+<p>&nbsp;</p>