题目描述：从上往下打印出二叉树的每个节点，同层节点从左至右打印。

输入描述：

    	    8
    	   /  \
    	  6   10
    	 / \  / \
    	5  7 9  11
输出：8 6 10 5 7 9 11

C++代码 :

struct TreeNode {
	int val;
	struct TreeNode *left;
	struct TreeNode *right;
	TreeNode(int x) :
			val(x), left(NULL), right(NULL) {
	}
};
class Solution {
public:
	vector<int> PrintFromTopToBottom(TreeNode* root) {

		vector<int> res;
		if (root == NULL)
			return res;
		queue<TreeNode*> q;
		q.push(root);
		while (!q.empty()) {
			res.push_back(q.front()->val);
			if (q.front()->left != NULL)
				q.push(q.front()->left);
			if (q.front()->right != NULL)
				q.push(q.front()->right);
			q.pop();
		}
		return res;
	}
};