32.1 从上往下打印二叉树

https://www.nowcoder.com/practice/7fe2212963db4790b57431d9ed259701?tpId=13&tqId=11175&tPage=1&rp=1&ru=%2Fta%2Fcoding-interviews&qru=%2Fta%2Fcoding-interviews%2Fquestion-ranking&from=cyc_github&tab=answerKey

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