面试题23：二叉树：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。

思路：
这道题实质是考查树的遍历算法。从上到下打印二叉树的规律：每一次打印一个结点的时候，如果该结点有子结点， 则把该结点的子结点放到一个队列的末尾。接下来到队列的头部取出最早进入队列的结点，重复前面的打印操作，直至队列中所有的结点都被打印出来为止。
广度优先遍历。

import java.util.ArrayList;import java.util.LinkedList;/**public class TreeNode {    int val = 0;    TreeNode left = null;    TreeNode right = null;    public TreeNode(int val) {        this.val = val;    }}*/public class Solution {    private ArrayList<Integer> list = new ArrayList<>();    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {        if(root == null) {            return new ArrayList<>();        }            LinkedList<TreeNode> linkedList = new LinkedList<>();        linkedList.add(root);        PrintFromTopToBottom(linkedList);        return list;    }    private void PrintFromTopToBottom(LinkedList<TreeNode> linkedList) {        if(linkedList == null || linkedList.isEmpty()) {            return;        }        while(!linkedList.isEmpty()) {            TreeNode head = linkedList.poll();            list.add(head.val);            if(head.left != null) {                linkedList.add(head.left);            }            if(head.right != null) {                linkedList.add(head.right);            }        }    }}

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涉及知识：

• 广度优先遍历
• 队列操作：LinkedList

public class LinkedList<E>    extends AbstractSequentialList<E>    implements List<E>, Deque<E>, Cloneable, java.io.Serializable

package java.util;public interface Queue<E> extends Collection<E> {    boolean add(E e);    /**     * Inserts the specified element into this queue if it is possible to do     * so immediately without violating capacity restrictions.     * When using a capacity-restricted queue, this method is generally     * preferable to {@link #add}, which can fail to insert an element only     * by throwing an exception.     *     * @param e the element to add     * @return {@code true} if the element was added to this queue, else     *         {@code false}     * @throws ClassCastException if the class of the specified element     *         prevents it from being added to this queue     * @throws NullPointerException if the specified element is null and     *         this queue does not permit null elements     * @throws IllegalArgumentException if some property of this element     *         prevents it from being added to this queue     */    boolean offer(E e);    /**     * Retrieves and removes the head of this queue.  This method differs     * from {@link #poll poll} only in that it throws an exception if this     * queue is empty.     *     * @return the head of this queue     * @throws NoSuchElementException if this queue is empty     */    E remove();    /**     * Retrieves and removes the head of this queue,     * or returns {@code null} if this queue is empty.     *     * @return the head of this queue, or {@code null} if this queue is empty     */    E poll();    /**     * Retrieves, but does not remove, the head of this queue.  This method     * differs from {@link #peek peek} only in that it throws an exception     * if this queue is empty.     *     * @return the head of this queue     * @throws NoSuchElementException if this queue is empty     */    E element();    /**     * Retrieves, but does not remove, the head of this queue,     * or returns {@code null} if this queue is empty.     *     * @return the head of this queue, or {@code null} if this queue is empty     */    E peek();}