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) {
        vector<vector<int>> res;
        if(!root) return res;
        queue<TreeNode*> q;
        q.push(root);
        q.push(nullptr);

        vector<int> line;
        int flag = 1;
        while(!q.empty()){
            auto t = q.front();
            q.pop();
            if(!t){
                //偶数
                if(!(flag & 1)) reverse(line.begin(), line.end());
                flag ++ ;
                res.push_back(line);   
                line.clear();
                if(q.size()) q.push(nullptr);//加if是为了判断最后一个nullptr
            }else{
                line.push_back(t->val);
                if(t->left) q.push(t->left);
                if(t->right) q.push(t->right);
            }
        }
        return res;
    }
};