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.

即找出二叉查找树中最小的元素。

思路：找最小值，可以参考中序遍历，借助栈！每弹出一个元素，才增加栈中元素，不用马上遍历整颗树!

实现代码：

class BSTIterator {
stack<TreeNode *> myStack;
public:
BSTIterator(TreeNode *root) {
TreeNode *node=root;
while(node!=NULL)
{
myStack.push(node);
node = node->left;
}
}

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

/** @return the next smallest number */
int next() {
TreeNode *tmpNode = myStack.top();
int res = tmpNode->val;
myStack.pop();

tmpNode=tmpNode->right;
while(tmpNode!=NULL)
{
myStack.push(tmpNode);
tmpNode = tmpNode->left;
}
return res;
}
};