经典题，需要记忆，且注意fabs和fmax函数的使用

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     struct TreeNode *left;*     struct TreeNode *right;* };*/int deep(struct TreeNode*root){if(root==NULL){return 0;}return fmax( deep(root->left),deep(root->right))+1;}
bool isBalanced(struct TreeNode* root) {if(root==NULL){return true;}if(fabs(deep(root->left)-deep(root->right))>1){return false;}if(isBalanced(root->left)&&isBalanced(root->right)){return true;}return false;
}