剑指offer:面试题23、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。

代码示例

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

public class Offer23 {
    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);
        root.left.left = new TreeNode(4);
        root.left.right = new TreeNode(5);
        Offer23 testObj = new Offer23();
        System.out.println(testObj.printFromTopToBottom(root));
    }

    static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int val) {
            this.val = val;
        }
    }

    public List<Integer> printFromTopToBottom(TreeNode root) {
        Queue<TreeNode> queue = new LinkedList<>();
        List<Integer> res = new ArrayList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            int count = queue.size();
            for (int i = 0; i < count; i++) {
                TreeNode node = queue.poll();
                res.add(node.val);
                if (node.left != null) {
                    queue.offer(node.left);
                }

                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
        }
        return res;
    }
}