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<ArrayList<Integer>>();
        if (root == null) {
            return res;
        }
         //采用队列存储每层非空节点，每次循环从队列中取出一层节点处理并将子节点放入队列，直到队列为空
        Queue<TreeNode> queue = new ArrayDeque<TreeNode>();
        queue.add(root);
        while (!queue.isEmpty()) {
            ArrayList<Integer> dataLine = new ArrayList<Integer>();
            int n = queue.size();
            for (int i = 0; i < n; i++) {
                TreeNode curNode = queue.poll();
                dataLine.add(curNode.val);
                if (curNode.left != null) {
                    queue.add(curNode.left);
                }
                if (curNode.right != null) {
                    queue.add(curNode.right);
                }
            }
            res.add(dataLine);
        }
        return res;

    }
}

作者：牛客854832741号
链接：https://www.nowcoder.com/discuss/527524992351858688
来源：牛客网