BST级订单遍历

Question

我想按级别顺序遍历打印BST。但是我以这种奇怪的方式得到了输出。另外，我使用Java Visualizer来检查算法，但由于Visualizer不能说明多个实例，所以我一无所知。我认为这是我的变量tmp没有正确添加到ArrayList实例set实例中，还是set没有正确添加到ArrayList<ArrayList<Integer>> solution中。 / p>

import java.util.LinkedList;
import java.util.Queue;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Arrays;

public class BSTordertraversal{

  public static class TreeNode{
    int value;
    TreeNode left;
    TreeNode right;
    public TreeNode(int value){
      this.value=value;
    }
  }
  public static ArrayList<ArrayList<Integer>> levelOrder(TreeNode root){
    HashMap<TreeNode,Integer> map = new HashMap<>();
    Queue<TreeNode> queue = new LinkedList<TreeNode>();
    ArrayList<Integer> set = new ArrayList<>();
    ArrayList<ArrayList<Integer>> solution = new ArrayList<ArrayList<Integer>>();
    queue.add(root);
    int lastlvl = 0;
    map.put(root,0);
    if(root==null){
      return solution;
    }
    int tmp;
    while(queue.peek()!=null){
      TreeNode current = queue.peek();
      if(current.left!=null){
        queue.add(current.left);
        map.put(current.left, map.get(current)+1);
      }
      if(current.right!=null){
        queue.add(current.right);
        map.put(current.right, map.get(current)+1);
      }
      if(map.get(current)==lastlvl){
        tmp = current.value;
        set.add(tmp);
        queue.remove();
      }else if(map.get(current)>lastlvl){
        solution.add(set);
        set.clear();
        tmp = current.value;
        set.add(tmp);
        lastlvl=map.get(current);
        queue.remove();
      }
    }
    return solution;
  }

  public static void main(String[] args) {
    TreeNode root = new TreeNode(3);
    root.left = new TreeNode(9);
    root.right = new TreeNode(20);
    //root.left.left = new TreeNode(null);
    //root.left.right= new TreeNode(null);
    root.right.left = new TreeNode(15);
    root.right.right= new TreeNode(7);
    //root.right.right.right = new TreeNode(30);
    ArrayList<ArrayList<Integer>> solution = levelOrder(root);
    System.out.println("[");
    for(int i=0;i<solution.size();i++){
      System.out.print("[");
      for(int x=0;x<solution.get(i).size();x++){
        System.out.print(solution.get(i).get(x));
      }
      System.out.print("]");
    }
    System.out.println("");
    System.out.println("]");
  }
}

以下是输出：

Answer 1

我将您的功能levelOrder稍微更改为：

 public static ArrayList<Integer> levelOrder(TreeNode root){
        Queue<TreeNode> queue=new LinkedList<TreeNode>();
        ArrayList<Integer> levelOrderData =  new ArrayList<>();
        queue.add(root);
        while(!queue.isEmpty())
        {
            TreeNode tempNode=queue.poll();
            levelOrderData.add(tempNode.value);
            if(tempNode.left!=null)
                queue.add(tempNode.left);
            if(tempNode.right!=null)
                queue.add(tempNode.right);
        }
        return levelOrderData;
  }

这是不必要的：

  ArrayList<ArrayList<Integer>> solution = new ArrayList<ArrayList<Integer>>();