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) {
       //层次遍历，使用队列
       ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();
       Queue<TreeNode> q = new ArrayDeque<TreeNode>();
       if(root == null) return list;
       q.add(root);
       while(!q.isEmpty()){
		 //该列表用来存储每一层数据
           ArrayList<Integer> list_1 = new ArrayList<Integer>();
		 //提前取队列长度，不能直接q.size()放进for
           int n = q.size();
           for(int i = 0; i < n; i++){
               TreeNode t = q.poll();
               list_1.add(t.val);
               if(t.left != null) q.add(t.left);
               if(t.right != null) q.add(t.right);
           }
		 //将一层数据放进list
           list.add(list_1);
       }
       return list;
    }
}