层次遍历

#include <vector>
class Solution {
public:
    vector<int> PrintFromTopToBottom(TreeNode* root) {
		if(root == nullptr) return {};
		
		std::queue<TreeNode*> Q;
		std::vector<int> res;
		Q.push(root);
		while(!Q.empty()){
			TreeNode* curP = Q.front();
			Q.pop();
			res.push_back(curP->val);
			if(curP->left) Q.push(curP->left);
			if(curP->right) Q.push(curP->right);
		}
		return res;
    }
};