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
        ArrayList<ArrayList<Integer>> res = new ArrayList();
        if(root ==null) return res;
        Queue<TreeNode> q = new ArrayDeque<TreeNode>();
        q.add(root);
        while(!q.isEmpty()){
            ArrayList<Integer> list = new ArrayList();
            int n = q.size();
            while(n-->0){
                TreeNode cur =q.poll();
                list.add(cur.val);
                if(cur.left!= null){
                    q.add(cur.left);
                }
                if(cur.right!= null){
                    q.add(cur.right);
                }
            }
            res.add(list);
        }


        return res;
    }
}

利用队列来 逐层存储 逐层遍历