[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