题目地址：

https://www.acwing.com/problem/content/42/

从上到下按层打印二叉树，同一层的结点按从左到右的顺序打印，每一层打印到一行。

直接BFS。代码如下：

#include <iostream>
#include <vector>
#include <queue>
using namespace std;

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

class Solution {
public:
    vector<vector<int>> printFromTopToBottom(TreeNode* root) {
        vector<vector<int>> res;
        if (!root) return res;

        queue<TreeNode*> q;
        q.push(root);
        while (q.size()) {
            int size = q.size();
            vector<int> v;
            for (int i = 0; i < size; i++) {
                auto t = q.front(); q.pop();
                v.push_back(t->val);
                if (t->left) q.push(t->left);
                if (t->right) q.push(t->right);
            }

            res.push_back(v);
        }
        
        return res;
    }
};

时空复杂度 O ( n ) O(n) O(n)。