Binary Search Tree Iterator

Implement an iterator over a binary search tree (BST). Your iterator will be initialized with the root node of a BST.

Calling next() will return the next smallest number in the BST.

Note: next() and hasNext() should run in average O(1) time and uses O(h) memory, where h is the height of the tree.

采用中序遍历将节点压入栈中，由于要求存储空间为O（h），因此不能在一开始将所有节点全部压入，只是压入最左边一列。当取出一个节点时，压入其右子节点的所有左节点。

 /**

  * Definition for binary tree

  * struct TreeNode {

  *     int val;

  *     TreeNode *left;

  *     TreeNode *right;

  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

  * };

  */

 class BSTIterator {

 private:

     stack <TreeNode*> stk;

     int minres;

 public:

     BSTIterator(TreeNode *root) {

         while(root)

         {

             stk.push(root);

             root=root->left;

         }

     }

     /** @return whether we have a next smallest number */

     bool hasNext() {

         if(stk.empty())

             return false;

         TreeNode* top;

         top=stk.top();

         minres=top->val;

         stk.pop();

         TreeNode* cur=top->right;

         if(cur)

         {

             stk.push(cur);

             cur=cur->left;

             while(cur)

             {

                 stk.push(cur);

                 cur=cur->left;

             }

         }

         return true;

     }

     /** @return the next smallest number */

     int next() {

         return minres;

     }

 };

 /**

  * Your BSTIterator will be called like this:

  * BSTIterator i = BSTIterator(root);

  * while (i.hasNext()) cout << i.next();

  */

巴特西

Binary Search Tree Iterator

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