二叉树的层序遍历

struct TreeNode{
	int val;
	TreeNode* left;
	TreeNode* right;
	TreeNode(): val(0),left(nullptr), right(nullptr) {}
	TreeNode(int x): val(x), left(nullptr), right(nullptr) {}
	TreeNode(int x, TreeNode* left, TreeNode* right): val(x), left(left), right(right) {}
};

/*
层序遍历：创建一个辅助队列，利用队列先进先出的特点，使用广度优先遍历实现层序遍历
*/
class Solution{
public:
	vector<vector<int>> levelOrder(TreeNode* root)
	{
    	queue<TreeNode*> que;
    	if(root != nullptr) que.push(root);
    	vector<vector<int>> result;
		while(!que.empty())
		{
			int size = que.size();
			vetor<int> vec;
			for(int i = 0; i < size; i++)
			{
				TreeNode* node = que.front();
				que.pop();
				vec.push_back(node->val);
				
				if(node->left) que.push(node->left);
				if(node->right) que.push(node->right);
			}
			result.push_back(vec);
		}
		return result;
	}
};