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<ArrayList<Integer>>();
        }

        ArrayList<ArrayList<Integer>> ans = new ArrayList<>();

        Queue<TreeNode> q = new LinkedList<>();

        q.offer(root);

        while(!q.isEmpty()) {
            ArrayList<Integer> row = new ArrayList<Integer>();
            int size = q.size();
            
            int i = 0;

            while(i < size) {
                TreeNode node = q.poll();

                row.add(node.val);


                if(node.left != null) {
                    q.offer(node.left);
                }

                if(node.right != null) {
                    q.offer(node.right);
                }
                i++;
            }

            ans.add(row);


        }
        return ans;
    }
}

1、层次遍历，记录