struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode *pred;
    TreeNode() : val(0), left(nullptr), right(nullptr), pred(nullptr){}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr), pred(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right, TreeNode *pred) : val(x), left(left), right(right), pred(pred){}
};