二叉树的层序遍历，leetcode习题

public class LevelOrder {
        public List> levelOrder(TreeNode root) {
            List> ret = new ArrayList>();
            if (root == null) {
                return ret;
            }
			//利用队列做辅助结构
            Queue queue = new linkedList();
            queue.offer(root);

            while (!queue.isEmpty()) {

                List level = new ArrayList();
                int currentLevelSize = queue.size();

                while(currentLevelSize>0){
					//先将队列的队头取出
                    TreeNode node = queue.poll();
                    level.add(node.val);
                    //然后再将此节点的左右节点进行入队
                    if (node.left!= null)queue.offer(node.left);
                    if (node.right!= null)queue.offer(node.right);
                    //对队长进行减减
                    currentLevelSize--;

                }
                //然后将树此层的节点压入链表中
                ret.add(level);
            }

            return ret;
        }

    public static void main(String[] args) {

            TreeNode treeNode  = new TreeNode(1);
            treeNode.left = new TreeNode(2);
            treeNode.right = new TreeNode(3);
        treeNode.left.left =new TreeNode(4);
        treeNode.left.right = new TreeNode(5);

        List> list = new LevelOrder().levelOrder(treeNode);
        System.out.println(list);



    }



}

思路：利用队做辅助结构，因为层序遍历，是从左到右的顺序遍历，所以使用队，不使用栈，栈适合深度遍历。