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.

 

 

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.



这个题constructor之类的， 不会写， 不知道什么原理
 


/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {
    
    private Stack<TreeNode> stack; // 为啥是 private

    public BSTIterator(TreeNode root) {
        stack = new Stack<>(); //为啥在这new 一下？ 
        pushLeft(root);
        
        
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return (!stack.isEmpty());
        
        
    }

    /** @return the next smallest number */
    public int next() {
        int curVal = 0;
        if(hasNext()){
            TreeNode curNode = stack.pop();
            curVal = curNode.val;
            if(curNode.right != null){
                pushLeft(curNode.right);
            }
        }
        return curVal;
        //return curNode.val;
        // Line 37: error: cannot find symbol: variable curNode
    }
    
    private void pushLeft(TreeNode node){
        while(node != null){
            stack.push(node);
            node = node.left;
        }

    }
}



  ===========================
  写个BST的in-order iterator，要写的function有 next() 和 all(), all() return所有剩下的。


/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class BSTIterator {
    
    private Stack<TreeNode> stack;
   

    public BSTIterator(TreeNode root) {
        stack = new Stack<>();
        pushLeft(root);
        
        
        
    }

    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.isEmpty();
    }

    /** @return the next smallest number */
    public int next() {
        int curVal = 0;
        if(hasNext()){
            TreeNode curNode = stack.pop();
            curVal = curNode.val;
            if(curNode.right != null){
                pushLeft(curNode.right);
            }
        }
        return curVal;
        //return curNode.val;
        // Line 37: error: cannot find symbol: variable curNode
    }
    
    public List<TreeNode> all(){
        List<TreeNode> result = new ArrayList<>();
        while(hasNext()){
            result.add(next());
        }
        return result;
        
    }
    
    private void pushLeft(TreeNode node){
        while(node != null){
            stack.push(node);
            node = node.left;
        }

    }
}





==============================
// preorder traversal
// 面试怎么做？

// 把 root 先放进 stack 里，pop stack, add it 
// to the list , 然后放 right ， 然后放 left
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
      
      List<Integer> result = new ArrayList<>();
      if(root == null) return result; // null 
      Stack<TreeNode> stack = new Stack<>();
      stack.push(root);
      
      while(!stack.isEmpty()){
        TreeNode cur = stack.pop();
        result.add(cur.val);
        if(cur.right != null) stack.push(cur.right);
        if(cur.left != null) stack.push(cur.left);
      }
      return result;
    }
}


==============================
// preorder iterator 
// 得把这个要自己定义的东西 搞懂， 记住， 现在还不会定义
  
// 
  
public class PreorderIterator{
  
  private Stack<TreeNode> stack;
  
  public PreorderIterator(TreeNode root){
    stack = new Stack<TreeNode>();
    if(root != null){
      stack.push(root);
    }
    
  }
  
  @Override
  public boolean hasNext(){
    return !stack.isEmpty();
  }
  
  @Override
  public int next(){
    if(!hasNext()){
      throw new NoSuchElementException("no nodes left");
    }
    
    TreeNode cur = stack.pop();
    if(cur.right != null) stack.push(cur.right);
    if(cur.left != null) stack.push(cur.left);
    return cur.val;
  }
  
}

========================================

// in order traversal 
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
      List<Integer> result = new ArrayList<>();
      if(root == null) return result;
      
      Stack<TreeNode> stack = new Stack<>();
      
      pushLeft(stack, root);
      
      while(!stack.isEmpty()){
        TreeNode cur = stack.pop();
        result.add(cur.val);
        if(cur.right != null){
          pushLeft(stack, cur.right);
        }
      }
      return result;
    }
    private void pushLeft(Stack<TreeNode> stack, TreeNode node){
      while(node != null){
        stack.push(node);
        node = node.left;
      }
        
    }
}