<LeetCode OJ> 173. 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.

分析：

感觉蛮简单的！就是一个中序遍历嘛！可能想的太简单了！

next()和hasnext()函数的空间复杂度为O(1)，时间复杂度为O(1)，这里并没有符合要求。

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class BSTIterator {public:    BSTIterator(TreeNode *root) {        inorderTraversal(root);//获取中序遍历结果        pos=0;    }    void inorderTraversal(TreeNode* root) {          if(root){                  inorderTraversal(root->left);                  result.push_back(root->val);                  inorderTraversal(root->right);              }          }          /** @return whether we have a next smallest number */    bool hasNext() {        return pos < result.size();    }    /** @return the next smallest number */    int next() {        return result[pos++];    }private:          vector<int> result;    int pos;};/** * Your BSTIterator will be called like this: * BSTIterator i = BSTIterator(root); * while (i.hasNext()) cout << i.next(); */

别人的算法设计：

class BSTIterator {    public:    BSTIterator(TreeNode *root) {        pushLeft(root);    }    /** @return whether we have a next smallest number */    bool hasNext() {        return !stk.empty();    }    /** @return the next smallest number */    int next() {//空间复杂度为O(h)，时间复杂度为O(h)        TreeNode *tmpNode = stk.top();        stk.pop();        pushLeft(tmpNode->right);        return tmpNode->val;    }        void pushLeft(TreeNode *node) {//空间复杂度为O(h)，时间复杂度为O(h)        while (node != NULL)        {            stk.push(node);            node = node->left;        }    }    private:    stack<TreeNode *> stk;};

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原文地址：http://blog.csdn.net/ebowtang/article/details/51660101

原作者博客：http://blog.csdn.net/ebowtang

本博客LeetCode题解索引：http://blog.csdn.net/ebowtang/article/details/50668895