```
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
int maxDepth(TreeNode* root) {
if(root==NULL)return 0;
if(root->left == NULL && root->right ==NULL) return 1;
int x = maxDepth(root->left);
int y = maxDepth(root->right);
if(x==0)return y+1;
if(y==0)return x+1;
return x>y?x+1:y+1;
}
public:
bool isBalanced(TreeNode* root) {
if(root == NULL)return true;
return abs(maxDepth(root->left)-maxDepth(root->right))<=1 && isBalanced(root->left) && isBalanced(root->right);
}
};
```

I found: a tree is balance means:

1: the depth ( max depth) differ between left and right child is no bigger than 1

2: left and right child is balance tree too.