认真的人改变自己，执着的人改变命运

                                                —— 24.9.27

102. 二叉树的层序遍历

示例 1：

输入：root = [3,9,20,null,null,15,7]
输出：[[3],[9,20],[15,7]]

示例 2：

输入：root = [1]
输出：[[1]]

示例 3：

输入：root = []
输出：[]

思路

对二叉树每一层进行遍历，知道遍历到每一个结点的叶子节点为止

叶子节点即左孩子右孩子都为null时的节点是叶子节点

从左到右每层进行遍历，每遍历一个节点，将节点存入队列中，最终返回队列

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        LinkedList queue = new LinkedList();
        queue.offer(root);
        int count1 = 1;
        while (!queue.isEmpty()) {
            List<Integer> level = new ArrayList<>();
            int count2 = 0;
            for (int i = 0; i < count1; i++) {
                TreeNode node = (TreeNode) queue.poll();
                level.add(node.val);
                if (node.left != null) {
                    queue.offer(node.left);
                    count2++;
                }
                if (node.right != null) {
                    queue.offer(node.right);
                    count2++;
                }
            }
            res.add(level);
            count1 = count2;
        }
        return res;
    }
}