Binary Search Tree Iterator ***

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class BSTIterator {
11 public:
12     stack<TreeNode* > stk;
13     BSTIterator(TreeNode *root) {
14         while(root){
15             stk.push(root);
16             root = root->left;
17         }
18     }
19 
20     /** @return whether we have a next smallest number */
21     bool hasNext() {
22         return !stk.empty();
23     }
24 
25     /** @return the next smallest number */
26     int next() {
27         TreeNode* temp = stk.top();
28         stk.pop();
29         int nextMin = temp->val;
30         temp = temp->right;
31         while(temp){
32             stk.push(temp);
33             temp = temp->left;
34         }
35         return nextMin;
36     }
37 };
38 
39 /**
40  * Your BSTIterator will be called like this:
41  * BSTIterator i = BSTIterator(root);
42  * while (i.hasNext()) cout << i.next();
43  */