题解 | #求二叉树的层序遍历#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
         if (root == null) {
            return new ArrayList<>();
        }
        Queue<TreeNode> que = new LinkedList<>();
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();

        que.add(root);
        while(!que.isEmpty()){
            int size = que.size();
            ArrayList<Integer> tmp = new ArrayList<>();
            for(int i = 0;i<size;i++){
                TreeNode t = que.remove();
                tmp.add(t.val);
                if(t.left!=null){
                    que.add(t.left);
                }
                if(t.right!=null){
                    que.add(t.right);
                }
            }
            res.add(tmp);

        }
        return res;
    }
}