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.

初始化的时候连续左孩子压栈直到压入最小节点，调用next的时候栈顶弹出，到栈顶右子树，重复初始化步骤，连续压入左孩子，保证栈顶元素总是下一个最小的。

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