【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=-;
     }

     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()-) 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();
  */