Leetcode 173. 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.

s思路：
1. 遍历树，用iterative。in-order遍历的套路。

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