题意：

树的层次遍历，基础数据结构题。

代码

struct TreeNode {
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

struct Node {
	TreeNode *TN;
	int L;
	Node(TreeNode *t, int i) :TN(t), L(i) {};
};

vector<vector<int>> levelOrder(TreeNode* root) {
	list<Node> li;
	Node p = Node(root, 0);
	li.push_back(p);
	int lv = 0;
	vector<vector<int>> res;
	vector<int> cur;
	
	while (!li.empty()) {
		p = li.front();
		li.pop_front();
		if (p.L != lv) {
			res.push_back(cur);
			cur.clear();
			lv++;
		}
		cur.push_back(p.TN->val);
		if (p.TN->left)
			li.push_back(Node(p.TN->left, lv + 1));
		if (p.TN->right)
			li.push_back(Node(p.TN->right, lv + 1));
	}
	res.push_back(cur);
	return res;
}