分行从上往下打印二叉树

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。

样例

输入如下图所示二叉树[8, 12, 2, null, null, 6, null, 4, null, null, null]
    8
   / \
  12  2
     /
    6
   /
  4

输出：[[8], [12, 2], [6], [4]]

/**
 * 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) {
        vector<vector<int>> res;
        if(!root) return res;
        
        queue<TreeNode*> q;
        q.push(root);
        q.push(nullptr);
        
        vector<int> level;
        
        while(q.size()){
            auto t = q.front();
            q.pop();
            
            if(!t){
                if(level.empty()) break;
                res.push_back(level);
                level.clear();
                q.push(nullptr);
                continue;
            }
            
            level.push_back(t->val);
            if(t->left) q.push(t->left);
            if(t->right) q.push(t->right);
        }
        
        return res;
    }
};