Binary Search Tree Iterator -- leetcode

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.

基本思路：

其实就是中序遍历的非递实现。

这next smallest意思就是，从小到大逐个返回。 第一次看到，竟然理解反了。还以为是从大到小逐个返回。

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class BSTIterator {public:    BSTIterator(TreeNode *root) {        while (root) {            s_.push(root);            root = root->left;        }    }    /** @return whether we have a next smallest number */    bool hasNext() {        return !s_.empty();    }    /** @return the next smallest number */    int next() {        auto root = s_.top();        s_.pop();        auto ans = root->val;        root = root->right;        while (root) {            s_.push(root);            root = root->left;        }        return ans;    }private:    stack<TreeNode *> s_;};/** * Your BSTIterator will be called like this: * BSTIterator i = BSTIterator(root); * while (i.hasNext()) cout << i.next(); */