【每日一题】leetcode 二叉树层序遍历 - 介绍

遍历顺序为：
F B G A D I C E H
一层一层遍历

import java.util.ArrayList;
import java.util.linkedList;
import java.util.List;
import java.util.Queue;
// 层次遍历
class Solution {
    public List> levelOrder(TreeNode root) {
        List> ret = new ArrayList>();
        if (root == null) {
            return ret;
        }

        Queue queue = new linkedList();
        queue.offer(root);
        while (!queue.isEmpty()) {
            List level = new ArrayList();
            int currentLevelSize = queue.size();
            for (int i = 1; i <= currentLevelSize; ++i) {
                TreeNode node = queue.poll();
                level.add(node.val);
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            ret.add(level);
        }
        
        return ret;
    }

    public static void main(String[] args) {
        TreeNode node = new TreeNode(3);
        TreeNode node3 = new TreeNode(9);
        TreeNode node4 = new TreeNode(20);

        TreeNode node2 = null;
        TreeNode node5 = null;
        TreeNode node15 = new TreeNode(15);
        TreeNode node7 = new TreeNode(7);
        node.left=node3;
        node.right=node4;
        node.right.left=node15;
        node.right.right=node7;

        List> lists = new Solution().levelOrder(node);
        System.out.println(lists);

    }
}

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