is_balanced_top_down.cpp

//T(n) = O(n)
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;
        } else {
            return max (leftHeight, rightHeight) + 1;
        }
}

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