Binary Search Tree Iterator —— Leetcode

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.

设计一个迭代器，关键在于O(h)的空间复杂度，用一个栈实现。

/**
* 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) {
s.push(root);
root = root->left;
}
}

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

/** @return the next smallest number */
int next() {
TreeNode* node = s.top();
s.pop();
if(node->right) {
TreeNode* rightNode = node->right;
s.push(rightNode);
while(rightNode->left) {
s.push(rightNode->left);
rightNode = rightNode->left;
}
}
return node->val;
}

private:
stack<TreeNode*> s;
};

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