【leetcode】Binary Search Tree Iterator

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.

最简单的，先把整棵树给遍历了，找到所有的值

 

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class BSTIterator {
public:
 
    vector<int> vals;
    int index;
    BSTIterator(TreeNode *root) {
        DFS(root);
        index=-1;
    }
   
    void DFS(TreeNode *root)
    {
       
        if(root==NULL)  return;
 
        DFS(root->left);
        vals.push_back(root->val);
        DFS(root->right);
    }
 
    /** @return whether we have a next smallest number */
    bool hasNext() {
       
        if(index<(int)vals.size()-1) return true;
        else return false;
 
    }
 
    /** @return the next smallest number */
    int next() {
        return vals[++index];
    }
};
 
/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = BSTIterator(root);
 * while (i.hasNext()) cout << i.next();
 */

 

 

运用二叉排序树的性质

 

首先连同root把所有左边节点都push到堆栈中，

然后每当调用next时，返回堆栈栈顶元素，然后把栈顶元素的右孩子，以及他的所有左子树都压入栈即可

 

/**
 * 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*> stk;
    BSTIterator(TreeNode *root) {
        pushLeftNode(root);
       
    }
   
    void pushLeftNode(TreeNode *root)
    {
 
        while(root!=NULL)
        {
            stk.push(root);
            root=root->left;
        }
    }
   
 
 
    /** @return whether we have a next smallest number */
    bool hasNext() {
       
        return !stk.empty();
    }
 
    /** @return the next smallest number */
    int next() {
       
        TreeNode * smallestNode=stk.top();
        stk.pop();
        pushLeftNode(smallestNode->right);
        return smallestNode->val;
    }
};
 
/**
 * Your BSTIterator will be called like this:
 * BSTIterator i = BSTIterator(root);
 * while (i.hasNext()) cout << i.next();
 */