【leetcode】Binary Search Tree Iterator

 1 class BSTIterator {
 2     stack<TreeNode *>s;
 3 public:
 4     BSTIterator(TreeNode *root) {
 5         while(!s.empty())
 6             s.pop();
 7         while(root)
 8         {
 9             s.push(root);
10             root=root->left;
11         }
12     }
13 
14     /** @return whether we have a next smallest number */
15     bool hasNext() {
16         return !s.empty();
17     }
18 
19     /** @return the next smallest number */
20     int next() {
21         TreeNode *tmp=s.top();
22         s.pop();
23         int ret=tmp->val;
24         tmp=tmp->right;
25         while(tmp)
26         {
27             s.push(tmp);
28             tmp=tmp->left;
29         }
30 
31         return ret;
32     }
33 };