Binary Search Tree Iterator

QUESTION

Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

1ST TRY

前序遍历，记录下左子树的遍历路径

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */

struct MyListNode {
    TreeNode* node;
    MyListNode *next;
    MyListNode(TreeNode* x) : node(x), next(NULL) {}
};
class BSTIterator {
public:
    BSTIterator(TreeNode *root) {
        currentRoot = root;
        leftTree = NULL;
        buildLeftTree(root);
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        if(leftTree) return true;
        else return false;
    }

    /** @return the next smallest number */
    int next() {
            ret = leftTree->node->val;
            tmpListNode = leftTree;
            leftTree = leftTree->next;
            buildLeftTree(tmpListNode->node->right);
            delete[] tmpListNode;
            return ret;
    
    }
    
    void buildLeftTree(TreeNode *root)
    {
        if(!root) return;
        tmpTreeNode = root;
        while(tmpTreeNode)
        {
            MyListNode *newListNode = new MyListNode(tmpTreeNode);
            newListNode->next = leftTree;
            leftTree = newListNode;
            tmpTreeNode = tmpTreeNode->left;
        }
    }
private:
    TreeNode *tmpTreeNode;
    TreeNode *currentRoot;
    MyListNode *tmpListNode;
    MyListNode *leftTree;
    int ret;
    
};

/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = BSTIterator(root);
 * while (i.hasNext()) cout << i.next();
 */

Result: Accepted