[leetcode][tree][stack] Binary Search Tree Iterator

题目：

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) {iter = root;while (iter){//一路向左sta.push(iter);iter = iter->left;}//此时栈顶节点是最左节点，也即最小节点}/** @return whether we have a next smallest number */bool hasNext() {return !sta.empty() || iter;}/** @return the next smallest number */ //最小数：中序遍历的第一节点int next() {iter = sta.top();sta.pop();int res = iter->val;iter = iter->right;while (iter){sta.push(iter);iter = iter->left;}//此时栈顶元素是最小值,为下一次做准备return res;}private:TreeNode *iter;stack<TreeNode *> sta;};/*** Your BSTIterator will be called like this:* BSTIterator i = BSTIterator(root);* while (i.hasNext()) cout << i.next();*/

注：此题实质是二叉树的中序遍历