NC15 求二叉树的层序遍历

给定一个二叉树，返回该二叉树层序遍历的结果，（从左到右，一层一层地遍历）
例如：
给定的二叉树是{3,9,20,#,#,15,7},

该二叉树层序遍历的结果是
[
[3],
[9,20],
[15,7]
]

提示:

0 <= 二叉树的结点数 <= 1500

使用队列和普通的层次遍历不同，这个题目需要存储结果

import java.util.*;

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

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

public class Solution {
    public static void main(String[] args) {
        Solution solution = new Solution();
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(4);
        root.right.left = new TreeNode(6);
        root.right.right = new TreeNode(7);
        root.left.left = new TreeNode(5);
        root.left.right = new TreeNode(3);
        ArrayList> res = solution.levelOrder(root);
        System.out.println(res.toString());
    }
    public ArrayList> levelOrder(TreeNode root) {
        // write code here
        ArrayList> list = new ArrayList<>();
        if (root == null) {
            return list;
        }
        Queue queue = new linkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            ArrayList subList = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
                subList.add(node.val);
            }
            list.add(subList);
        }
        return list;
    }
}

普通的层次遍历比较的简单

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

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

public class Solution {
    private List list = new ArrayList<>();
    public static void main(String[] args) {
        Solution solution = new Solution();
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(4);
        root.right.left = new TreeNode(6);
        root.right.right = new TreeNode(7);
        root.left.left = new TreeNode(5);
        root.left.right = new TreeNode(3);
        List list = solution.doSolution(root);
        System.out.println(list.toString());
    }

    private List doSolution(TreeNode root) {
        if (root == null) {
            return new ArrayList<>();
        }
        Queue queue = new linkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            list.add(node.val);
            if (node.left != null) {
                queue.add(node.right);
            }
            if (node.right != null) {
                queue.add(node.left);
            }
        }
        return list;
    }
}

输出的是这样的格式