LeetCode - Binary Search Tree Iterator

Binary Search Tree Iterator

 Total Accepted: 20470 Total
Submissions: 69577My Submissions

Question
 Solution 

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.

s

class BSTIterator {
public:
BSTIterator(TreeNode *root) {
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* now = stk.top();
int ret = now->val;
stk.pop();
if(now->right){
now = now->right;
while(now){
stk.push(now);
now = now->left;
}
}
return ret;
}
private:
stack<TreeNode*> stk;
};