Binary Search Tree Iterator

 1 /**
 2  * Definition of TreeNode:
 3  * class TreeNode {
 4  * public:
 5  *     int val;
 6  *     TreeNode *left, *right;
 7  *     TreeNode(int val) {
 8  *         this->val = val;
 9  *         this->left = this->right = NULL;
10  *     }
11  * }
12  * Example of iterate a tree:
13  * BSTIterator iterator = BSTIterator(root);
14  * while (iterator.hasNext()) {
15  *    TreeNode * node = iterator.next();
16  *    do something for node
17  */
18 class BSTIterator {
19 public:
20     //@param root: The root of binary tree.
21     BSTIterator(TreeNode *root) {
22         // write your code here
23         inorder = inorderTraversal(root);
24         index = 0;
25     }
26 
27     //@return: True if there has next node, or false
28     bool hasNext() {
29         // write your code here
30         return index < inorder.size();
31     }
32     
33     //@return: return next node
34     TreeNode* next() {
35         // write your code here
36         if (index >= inorder.size()) return NULL;
37         return inorder[index++];
38     }
39     
40 private:
41     vector<TreeNode* > inorder;
42     int index;
43     
44     vector<TreeNode* > inorderTraversal(TreeNode* root) {
45         vector<TreeNode* > result;
46         
47         helper(result, root);
48         return result;
49     }
50     
51     void helper(vector<TreeNode* >& result, TreeNode* root) {
52         if (!root) return;
53         
54         helper(result, root->left);
55         result.push_back(root);
56         helper(result, root->right);
57     }
58 };