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。

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

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

}

}
*/
import java.util.*;
public class Solution {
public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
Queue<TreeNode> queue = new LinkedList<TreeNode>();
ArrayList<Integer> list = new ArrayList<Integer>();
if(root == null){
return list;
}
queue.add(root);
while(queue.peek() != null){
root = queue.poll();
list.add(root.val);
if(root.left != null)
queue.add(root.left);
if(root.right != null)
queue.add(root.right);
}
return list;
}
}