题目链接https://leetcode.cn/problems/binary-tree-level-order-traversal/description/

通过队列实现层序遍历

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