牛客网-剑指office-从上到下打印二叉树

题目：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。
思路：层次遍历

/*
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) {
        if(!root) return {};
		vector<int> ans;
		deque<TreeNode*> d;
		d.push_back(root);
		while (!d.empty())
		{
			TreeNode* t = d.front();
			d.pop_front();
			ans.push_back(t->val);
			if (t->left != nullptr) d.push_back(t->left);
			if (t->right != nullptr) d.push_back(t->right);
		}
		return ans;
	}
};