题解 | #求二叉树的层序遍历#

看一个节点有没有子节点，有子节点按左右顺序入队列，然后记录该节点的值

import java.util.*;

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

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        Queue<TreeNode> q = new LinkedList();
        q.offer(root);
        if(root == null)
            return res;
        while(!q.isEmpty()){
            ArrayList<Integer> arr = new ArrayList<>();
            int count = q.size();
            while(count-- > 0){
                TreeNode tree = q.poll();
                if(tree.left != null){
                    q.offer(tree.left);
                }
                if(tree.right != null){
                    q.offer(tree.right);
                }
                arr.add(tree.val);
            }
            res.add(arr);
        }
        return res;
    }
}