/**
* 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){
st.push(root);
root = root->left;
}
}
/** @return whether we have a next smallest number */
bool hasNext() {
return !st.empty();
}
/** @return the next smallest number */
int next() {
TreeNode* temp = st.top(); st.pop();
int res = temp->val;
if(temp->right){
temp = temp->right;
while(temp){
st.push(temp);
temp = temp->left;
}
}
return res;
}
private:
stack<TreeNode*> st;
};
/**
* Your BSTIterator will be called like this:
* BSTIterator i = BSTIterator(root);
* while (i.hasNext()) cout << i.next();
*/
find the next smallest
1.a.go right once
b.go left all the way down
2.go upwards