PrintTreeFromTopToBottom.java

package offer.problem32;

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;

/**
 * Created with IntelliJ IDEA
 *
 * @Author yuanhaoyue swithaoy@gmail.com
 * @Description 32.1. 从上往下打印二叉树（层次遍历二叉树）
 * @Date 2019-01-24
 * @Time 20:49
 */
public class PrintTreeFromTopToBottom {
    public ArrayList<Integer> printFromTopToBottom(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<>();
        if (root != null) {
            //使用队列辅助运算
            Queue<TreeNode> queue = new LinkedList<>();
            queue.add(root);
            while (!queue.isEmpty()) {
                TreeNode node = queue.poll();
                list.add(node.val);
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
        }
        return list;
    }

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

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