从上到下打印二叉树

题目一

题目一：不分行从上到下打印二叉树

分析

不分行从上到下打印二叉树

源码

public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<>();
        if (root==null){
            return result;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (queue.size()>0){
            TreeNode first = queue.poll();
            result.add(first.val);
            if(first.left!=null){
                queue.offer(first.left);
            }
            if(first.right!=null){
                queue.offer(first.right);
            }
        }
        return result;
    }
}

题目二

题目二：分行从上到下打印二叉树

分析

分行从上到下打印二叉树

源码

import java.util.ArrayList;
import java.util.*;
public class Solution {
    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer>> arrayLists = new ArrayList<>();
        if(pRoot==null){
            return arrayLists;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(pRoot);
        int printLeft=1;
        int toBePrint=0;
        ArrayList<Integer> result = new ArrayList<>();
        while (!queue.isEmpty()){
            TreeNode temp = queue.poll();
            if(temp.left!=null){
                queue.offer(temp.left);
                toBePrint++;
            }
            if(temp.right!=null){
                queue.offer(temp.right);
                toBePrint++;
            }
            if(printLeft>0){
                result.add(temp.val);
                printLeft--;
            }
            if(printLeft<=0){
                printLeft=toBePrint;
                toBePrint=0;
                arrayLists.add(result);
                result = new ArrayList<>();
            }
        }
        return arrayLists;
    }
    
}

题目三

题目：之字形打印二叉树

分析

之字形打印二叉树

源码

import java.util.*;
public class Solution {
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer>> lists = new ArrayList<>();
        if(pRoot==null){
            return lists;
        }
        Stack<TreeNode> stack1 = new Stack<>();//奇数层使用的栈
        Stack<TreeNode> stack2 = new Stack<>();//偶数层使用的栈
        int cur=0;
        int next=1;
        stack1.push(pRoot);
        while (!stack1.isEmpty()||!stack2.isEmpty()){
            if(cur==0&&!stack1.isEmpty()){
                ArrayList<Integer> list = new ArrayList<>();
                while (!stack1.isEmpty()){
                    TreeNode temp=stack1.pop();
                    list.add(temp.val);
                    if(temp.left!=null){
                        stack2.push(temp.left);
                    }
                    if(temp.right!=null){
                        stack2.push(temp.right);
                    }
                }
                lists.add(list);
            }
            else if (cur==1&&!stack2.isEmpty()){
                ArrayList<Integer> list = new ArrayList<>();
                while (!stack2.isEmpty()){
                    TreeNode temp=stack2.pop();
                    list.add(temp.val);
                    if(temp.right!=null){
                        stack1.push(temp.right);
                    }
                    if(temp.left!=null){
                        stack1.push(temp.left);
                    }
                }
                lists.add(list);
            }
            cur=1-cur;
            next=1-next;
        }
        return lists;
    }

}