力扣【102】二叉樹的層序周遊

題目：（vivo面試）

給你一個二叉樹，請你傳回其按 層序周遊 得到的節點值。 （即逐層地，從左到右通路所有節點）。

示例：

二叉樹：[3,9,20,null,null,15,7],

    3

   / \

  9  20

    /  \

   15   7

傳回其層次周遊結果：

[

  [3],

  [9,20],

  [15,7]

]

題解：

import java.util.*;
class TreeNode {
    int val;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;
    }
}
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        //層序周遊用隊列
        //邊界條件
        List<List<Integer>> list = new ArrayList<>();
        if (root == null) {
            return list;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            List<Integer> subList = new ArrayList<>();//注意：新定義的subList一定要放到while裡面
            int size = queue.size();//size是queue的size
            for (int i = 0; i < size; i++) {
                TreeNode temp = queue.poll();
                subList.add(temp.val);
                if (temp.left != null) {
                    queue.add(temp.left);
                }
                if (temp.right != null) {
                    queue.add(temp.right);
                }
            }
            list.add(subList);
        }
        return list;
    }
}


public class Main {
    public static void main(String[] args) {
        TreeNode head = new TreeNode(1);//849
        TreeNode second = new TreeNode(2);//850
        TreeNode three = new TreeNode(3);//851
        TreeNode four = new TreeNode(4);//852
        TreeNode five = new TreeNode(5);//853
        TreeNode six = new TreeNode(6);//854
        TreeNode seven = new TreeNode(7);//855

        head.right = three;
        head.left = second;
        second.right = five;
        second.left = four;
        three.right = seven;
        three.left = six;

        Solution s = new Solution();
        List<List<Integer>> list = s.levelOrder(head);
        System.out.println(list);
    }
}