树的层次遍历

import java.util.ArrayList;
import java.util.Queue;
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 {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        if(root==null){
            return list;
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        //放入队列
        queue.offer(root);
        //当队列不为空
        while(!queue.isEmpty()){
            TreeNode treeNode = queue.poll();
            //如果左节点不为空，左节点放入队列
            if(treeNode.left != null){
                queue.offer(treeNode.left);
            }
            //如果右节点不为空，把右节点放入队列
            if(treeNode.right != null){
                queue.offer(treeNode.right);
            }
            list.add(treeNode.val);
        }
        return list;

    }
}