LeetCode – Refresh – Binary 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 private:
12     stack<TreeNode *> s;
13 public:
14     void initialNodes(TreeNode *root) {
15         while (root) {
16             s.push(root);
17             root = root->left;
18         }
19     }
20     BSTIterator(TreeNode *root) {
21         initialNodes(root);
22     }
23 
24     /** @return whether we have a next smallest number */
25     bool hasNext() {
26         return !s.empty();
27     }
28 
29     /** @return the next smallest number */
30     int next() {
31         TreeNode *tmp = s.top();
32         s.pop();
33         if (tmp->right) {
34             initialNodes(tmp->right);
35         }
36         return tmp->val;
37     }
38 };
39 
40 /**
41  * Your BSTIterator will be called like this:
42  * BSTIterator i = BSTIterator(root);
43  * while (i.hasNext()) cout << i.next();
44  */