102. 二叉树的层序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<>();

        if (root == null)
        {
            return ans;
        }

        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);

        while(!q.isEmpty())
        {
            List<Integer> tmp = new ArrayList<>();
            int len = q.size();

            for(int i = 0; i < len; i++)
            {
                TreeNode node = q.poll();  //新的结点等于啥？？？？？当然是弹出得了
                tmp.add(node.val);

                if(node.left != null)
                {
                    q.add(node.left);
                }
                if(node.right != null)
                {
                    q.add(node.right);
                }

            }
            ans.add(tmp); //不需要插在前面，顺序即可
        }
        return ans;
    }
}