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);
            }
        }
    }
}

---

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();
}