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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。
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*> stk; 13 int minres; 14 public: 15 BSTIterator(TreeNode *root) { 16 while(root) 17 { 18 stk.push(root); 19 root=root->left; 20 } 21 } 22 23 /** @return whether we have a next smallest number */ 24 bool hasNext() { 25 if(stk.empty()) 26 return false; 27 TreeNode* top; 28 top=stk.top(); 29 minres=top->val; 30 stk.pop(); 31 TreeNode* cur=top->right; 32 if(cur) 33 { 34 stk.push(cur); 35 cur=cur->left; 36 while(cur) 37 { 38 stk.push(cur); 39 cur=cur->left; 40 } 41 } 42 return true; 43 } 44 45 /** @return the next smallest number */ 46 int next() { 47 return minres; 48 } 49 }; 50 51 /** 52 * Your BSTIterator will be called like this: 53 * BSTIterator i = BSTIterator(root); 54 * while (i.hasNext()) cout << i.next(); 55 */
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原文地址：http://www.cnblogs.com/zl1991/p/4705921.html
