LeetCode *** 173. Binary Search Tree Iterator

题目：

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, whereh is the height of the tree.

分析：

中序遍历即可。

代码：

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class BSTIterator {public:    stack<TreeNode*> tree;        BSTIterator(TreeNode *root) {        while(root){            tree.push(root);            root=root->left;        }    }    /** @return whether we have a next smallest number */    bool hasNext() {        return !tree.empty();    }    /** @return the next smallest number */    int next() {        TreeNode* tmp=tree.top();        tree.pop();        TreeNode* right=tmp->right;                while(right){            tree.push(right);            right=right->left;        }                return tmp->val;    }};/** * Your BSTIterator will be called like this: * BSTIterator i = BSTIterator(root); * while (i.hasNext()) cout << i.next(); */