86. 二叉查找树迭代器

描述

设计实现一个带有下列属性的二叉查找树的迭代器：

• 元素按照递增的顺序被访问（比如中序遍历）
• next()和hasNext()的询问操作要求均摊时间复杂度是O(1)

样例

对于下列二叉查找树，使用迭代器进行中序遍历的结果为 [1, 6, 10, 11, 12]

挑战

额外空间复杂度是O(h)，其中h是这棵树的高度
Super Star：使用O(1)的额外空间复杂度

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 * Example of iterate a tree:
 * BSTIterator iterator = BSTIterator(root);
 * while (iterator.hasNext()) {
 *    TreeNode * node = iterator.next();
 *    do something for node
 */


class BSTIterator {
public:
    /*
    * @param root: The root of binary tree.
    */
    
    std::stack<TreeNode *>  stk;
    BSTIterator(TreeNode * root) {
        // do intialization if necessary
        while(root != NULL) {
            stk.push(root);
            root = root->left;
        }
    }

    /*
     * @return: True if there has next node, or false
     */
    bool hasNext() {
        // write your code here
        return !stk.empty();
    }

    /*
     * @return: return next node
     */
    TreeNode * next() {
        // write your code here
        TreeNode* ans = stk.top();
        stk.pop();
        TreeNode* tmp = ans->right;
        while (tmp != NULL) {
            stk.push(tmp);
            tmp = tmp->left;
        }
        return ans;
    }
};