1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42

class BSTIterator {
public:
    int pos;
    vector<TreeNode *> treeNodeStack;

    void buildStack(TreeNode *node)
    {
        if (node == NULL)
        {
            return;
        }
        else
        {
            if (NULL != node->left)
            {
                buildStack(node->left);
            }
            treeNodeStack.push_back(node);
            if (NULL != node->right)
            {
                buildStack(node->right);
            }
        }
    }

    BSTIterator(TreeNode *root) {
        pos = 0;
        treeNodeStack.clear();
        buildStack(root);
    }

    /** @return whether we have a next smallest number */
    bool hasNext() {
        return treeNodeStack.size() > pos;
    }

    /** @return the next smallest number */
    int next() {
        return treeNodeStack[pos++]->val;
    }

};