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+# [110. Balanced Binary Tree](https://leetcode.com/problems/balanced-binary-tree/description/)
+# 思路
+一棵非空树是平衡二叉树的充要条件是：
+* 其左右子树的高相差不超过1；
+* 且左右子树都是平衡二叉树。
+
+因此是一个递归算法。
+# C++
+``` C++
+class Solution {
+private:
+    int getDepth(TreeNode *root){
+        if(root == NULL) return 0;
+        return 1 + max(getDepth(root -> left), getDepth(root -> right));
+    }
+public:
+    bool isBalanced(TreeNode* root) {
+        if(root == NULL) return true;
+        return (abs(getDepth(root -> left) - getDepth(root -> right)) <= 1) && isBalanced(root -> left) && isBalanced(root -> right);
+    }
+};
+```