// bottom up approach
int dfsHeight(TreeNode* root){
if(!root) return 0;
int leftHeight = dfsHeight(root->left);
int rightHeight = dfsHeight(root->right);
if(leftHeight < 0 || rightHeight < 0 || abs(leftHeight - rightHeight) > 1)
return -1;
else return max(leftHeight, rightHeight) + 1;
}
//dfs approach, bottom up
int dfsHeight(TreeNode* root){
if(!root) return 0;
int left = dfsHeight(root->left);
if(left==-1) return -1;
int right = dfsHeight(root->right);
if(right==-1) return -1;
if(abs(left-right)>1) return -1;
return max(left,right)+1;
}
int dfsHeight(TreeNode *cur){
if(cur == NULL){return 0;}
int left = dfsHeight(cur -> left);
if(left == -1){return -1;}
int right = dfsHeight(cur -> right);
if(right == -1){return -1;}
if(abs(left - right) > 1){return -1;}
return max(left, right) + 1;
}
bool isBalanced(TreeNode *root) {
return dfsHeight (root) != -1;
}
