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 class Solution {
    /**
     *
     * @param root TreeNode类
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
        ArrayList<ArrayList<Integer>> levelOrder = new ArrayList<>();
        if (root == null)
            return levelOrder;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            Queue<TreeNode> children = new LinkedList<>();
            ArrayList<Integer> temp = new ArrayList<>();
            while (!queue.isEmpty()) {
                temp.add(queue.peek().val);
                if (queue.peek().left != null) {
                    children.add(queue.peek().left);
                }
                if (queue.peek().right != null) {
                    children.add(queue.peek().right);
                }
                queue.poll();
            }
            levelOrder.add(temp);
            while (!children.isEmpty()) {
                queue.add(children.poll());
            }
        }
        return levelOrder;
    }
}