[leetcode] Binary Search Tree Iterator

From : https://leetcode.com/problems/binary-search-tree-iterator/

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.

/** * 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) {        index = root;while(index) {house.push(index);index = index->left;}    }    /** @return whether we have a next smallest number */    bool hasNext() {        while(index) {house.push(index);index = index->left;}return !house.empty();    }    /** @return the next smallest number */    int next() {        int ans = house.top()->val;index = house.top()->right;house.pop();return ans;    }private:stack<TreeNode *> house;TreeNode *index;};/** * Your BSTIterator will be called like this: * BSTIterator i = BSTIterator(root); * while (i.hasNext()) cout << i.next(); */