binaryTreeIsBalanced.cpp

 bool isBalanced(TreeNode* root) {
        return dfsHeight (root) != -1;
    }

    int dfsHeight (TreeNode *root) {

        if (root == NULL) return 0;
        
        int leftHeight = dfsHeight (root -> left);

        if (leftHeight == -1) 
            return -1;
        
        int rightHeight = dfsHeight (root -> right);

        if (rightHeight == -1) 
            return -1;
        
        if (abs(leftHeight - rightHeight) > 1)  
            return -1;

        return max (leftHeight, rightHeight) + 1;
    }