解题思路使用队列class Solution { public ListListInteger levelOrder(TreeNode root) { if (root null) return new ArrayList(); DequeTreeNode queue new ArrayDeque(); ListListInteger res new ArrayList(); queue.add(root); while (!queue.isEmpty()) { int size queue.size(); ListInteger path new ArrayList(); for (int i 0; i size; i) { TreeNode tmp queue.poll(); if (tmp.left ! null) queue.add(tmp.left); if (tmp.right ! null) queue.add(tmp.right); path.add(tmp.val); } res.add(path); } return res; } }