Leetcode: 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.
中序遍历的思路。

用栈来保存从根到最左侧叶子节点的路径，栈最上面的结点是最小的结点，每次取next,都是取栈最上面的结点，然后把剩余结点到最左侧叶子节点的路径放入栈中。

/**
* Definition for binary tree
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class BSTIterator {
public:
stack<TreeNode*> stk;
BSTIterator(TreeNode *root) {
while(!stk.empty())
stk.pop();

while(root){
stk.push(root);
root = root->left;
}
}

/** @return whether we have a next smallest number */
bool hasNext() {
return !stk.empty();
}

/** @return the next smallest number */
int next() {
TreeNode* tmp = stk.top();
int res = tmp->val;
stk.pop();
tmp = tmp->right;
while(tmp)
{
stk.push(tmp);
tmp = tmp->left;
}
return res;
}
};

/**
* Your BSTIterator will be called like this:
* BSTIterator i = BSTIterator(root);
* while (i.hasNext()) cout << i.next();
*/