描述：
Given a binary tree, determine if it is height-balanced.
For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.
思路：
用两个递归，一个递归找出树的最大深度，用另一个递归判断是否是平衡二叉树
C++代码：
class Solution {
public:
bool isBalanced(TreeNode *root) {
if (root == NULL) { return true; }
if (abs(maxDepth(root->left) - maxDepth(root->right)) > 1) {
return false;
}
return isBalanced(root->left) and isBalanced(root->right);
}
int maxDepth(TreeNode *root) {
if (root == NULL) { return 0; }
return max(maxDepth(root->left), maxDepth(root->right)) + 1;
}
};
