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.

 

将非递归的中序遍历分为两部分，第一部分为将左子树上的结点全部压入栈中，第二部分为栈顶元素出栈，同时将栈顶元素的右子树上的结点压入栈中。

 

C++实现代码：

#include<iostream>

#include<new>

#include<stack>

using namespace std;



/**

 * 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*> st;

    BSTIterator(TreeNode *root)

    {

        while(root)

        {

            st.push(root);

            root=root->left;

        }

    }



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

    bool hasNext()

    {

        return !st.empty();

    }



    /** @return the next smallest number */

    int next()

    {

        TreeNode *tmp=NULL;

        if(!st.empty())

        {

            tmp=st.top();

            st.pop();

            TreeNode *cur=tmp->right;

            while(cur)

            {

                st.push(cur);

                cur=cur->left;

            }

        }

        return tmp->val;

    }

};



void insert(TreeNode *&root,int val)

{

    if(root==NULL)

    {

        root=new TreeNode(val);

    }

    else if(val<root->val)

        insert(root->left,val);

    else

        insert(root->right,val);

}



void createBST(TreeNode *&root)

{

    int i;

    int arr[10]= {2,4,6,1,3,5,9,8,7,10};

    for(i=0; i<10; i++)

    {

        insert(root,arr[i]);

    }

}



int main()

{

    TreeNode *root=NULL;

    createBST(root);

    BSTIterator s(root);

    for(int i=0;i<10;i++)

        cout<<s.next()<<" ";

    cout<<endl;

}

看看非递归的中序遍历。。