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
        if (root == null) {
            return new ArrayList<>();
        }

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

        ArrayDeque<TreeNode> deque = new ArrayDeque<>();
        deque.add(root);

        while (!deque.isEmpty()) {
            ArrayList<Integer> list = new ArrayList<>();
            int size = deque.size();
            for (int i = 0; i < size; i++) {
                TreeNode treeNode = deque.pop();
                list.add(treeNode.val);
                if (treeNode.left != null) {
                    deque.add(treeNode.left);
                }
                if (treeNode.right != null) {
                    deque.add(treeNode.right);
                }
            }
            lists.add(list);
        }

        return lists;
    }
}