86. 二叉查找树迭代器
描述
设计实现一个带有下列属性的二叉查找树的迭代器:
- 元素按照递增的顺序被访问(比如中序遍历)
- next()和hasNext()的询问操作要求均摊时间复杂度是O(1)
样例
对于下列二叉查找树,使用迭代器进行中序遍历的结果为 [1, 6, 10, 11, 12]
挑战
额外空间复杂度是O(h),其中h是这棵树的高度
Super Star:使用O(1)的额外空间复杂度
/**
* Definition of TreeNode:
* class TreeNode {
* public:
* int val;
* TreeNode *left, *right;
* TreeNode(int val) {
* this->val = val;
* this->left = this->right = NULL;
* }
* }
* Example of iterate a tree:
* BSTIterator iterator = BSTIterator(root);
* while (iterator.hasNext()) {
* TreeNode * node = iterator.next();
* do something for node
*/
class BSTIterator {
public:
/*
* @param root: The root of binary tree.
*/
std::stack<TreeNode *> stk;
BSTIterator(TreeNode * root) {
// do intialization if necessary
while(root != NULL) {
stk.push(root);
root = root->left;
}
}
/*
* @return: True if there has next node, or false
*/
bool hasNext() {
// write your code here
return !stk.empty();
}
/*
* @return: return next node
*/
TreeNode * next() {
// write your code here
TreeNode* ans = stk.top();
stk.pop();
TreeNode* tmp = ans->right;
while (tmp != NULL) {
stk.push(tmp);
tmp = tmp->left;
}
return ans;
}
};