22、从上往下打印二叉树

题目描述
从上往下打印出二叉树的每个节点，同层节点从左至右打印。

import java.util.ArrayList;
import java.util.*;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        ArrayList<Integer> list = new ArrayList<>();
        PrintFromTopToBottom(root,list,queue);
        return list;
    }
    
    public void PrintFromTopToBottom(TreeNode root, ArrayList<Integer> list, Queue<TreeNode> queue){
        if(root!=null){
            list.add(root.val);
            queue.offer(root.left);
            queue.offer(root.right);
        }
        if(!queue.isEmpty())
            PrintFromTopToBottom(queue.poll(),list,queue);
    }
}

2017.6.2 第二遍做

import java.util.*;

/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    Queue<TreeNode> q = new LinkedList<>();
    ArrayList<Integer> ret = new ArrayList<>();
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        if(root == null){
            return ret;
        }
        
        ret.add(root.val);
        q.add(root.left);
        q.add(root.right);
        while(!q.isEmpty()){
            PrintFromTopToBottom(q.remove());
        }
        return ret;
        
    }
}