C++ 代码

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> printFromTopToBottom(TreeNode* root) {
        if(!root) return {};
        queue<TreeNode*> q;
        q.push(root);
        vector<vector<int>> ret;
        while(!q.empty()){
            int len = q.size();
            vector<int> ans;
            while(len --){
                auto t = q.front();
                q.pop();
                ans.push_back(t->val);
                if(t->left) q.push(t->left);
                if(t->right) q.push(t->right);
            }
            ret.emplace_back(ans);
        }
        return ret;
    }
};