1586. Binary Search Tree Iterator II

Implement the BSTIterator class that represents an iterator over the in-order traversal of a binary search tree (BST):

• BSTIterator(TreeNode root) Initializes an object of the BSTIterator class. The root of the BST is given as part of the constructor. The pointer should be initialized to a non-existent number smaller than any element in the BST.
• boolean hasNext() Returns true if there exists a number in the traversal to the right of the pointer, otherwise returns false.
• int next() Moves the pointer to the right, then returns the number at the pointer.
• boolean hasPrev() Returns true if there exists a number in the traversal to the left of the pointer, otherwise returns false.
• int prev() Moves the pointer to the left, then returns the number at the pointer.

Notice that by initializing the pointer to a non-existent smallest number, the first call to next() will return the smallest element in the BST.

You may assume that next() and prev() calls will always be valid. That is, there will be at least a next/previous number in the in-order traversal when next()/prev() is called.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class BSTIterator {
    vector<int> A;
    int p = 0;
    void init(TreeNode* node) {
        if(!node) return;
        init(node->left);
        A.push_back(node->val);
        init(node->right);
    }
public:
    BSTIterator(TreeNode* root) {
        A.push_back(INT_MIN);
        init(root);
    }
    
    bool hasNext() {
        return p + 1 < A.size();
    }
    
    int next() {
        return A[++p];
    }
    
    bool hasPrev() {
        return p > 1;
    }
    
    int prev() {
        return A[--p];
    }
};

/**
 * Your BSTIterator object will be instantiated and called as such:
 * BSTIterator* obj = new BSTIterator(root);
 * bool param_1 = obj->hasNext();
 * int param_2 = obj->next();
 * bool param_3 = obj->hasPrev();
 * int param_4 = obj->prev();
 */