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<>>
     */
    // 典型的利用队列来实现二叉树的层序遍历
    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
        ArrayList<ArrayList<Integer>> result = new ArrayList<>();
        if(root == null) return result;
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        while(!q.isEmpty()) {
            ArrayList<Integer> temp = new ArrayList<>();
            int sz = q.size();
            for(int i = 0; i < sz; i++) {
                TreeNode node = q.poll();
                temp.add(node.val);
                if(node.left != null) q.offer(node.left);
                if(node.right != null) q.offer(node.right);
            }
            result.add(temp);
        }
        return result;
    }
}