Kth Smallest Element In Tree

int dfs(TreeNode* A, int& B) {
    if (!A) return -1;
    int candidate = dfs(A->left, B);
    if (B == 0) return candidate;
    B--;
    if (B == 0) return A->val;
    return dfs(A->right, B);
}

int Solution::kthsmallest(TreeNode* A, int B) {
    return dfs(A, B);
}