C++ 代码(简单易懂)

// 二叉树层序遍历的变形(null标记)
/**
 * 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 vector<vector<int>>();

        vector<vector<int>> res;
        vector<int> temp;
        queue<TreeNode*> q;
        q.push(root);
        q.push(nullptr);
        int curnum = 0;                       // 统计当前二叉树的层数 
        while(q.size())
        {
            auto cur = q.front();
            q.pop();
            if(cur)
            {
                temp.push_back(cur->val);
                if(cur->left) q.push(cur->left);
                if(cur->right) q.push(cur->right);
            }
            else 
            {
                curnum++;
                if(curnum % 2 == 1) res.push_back(temp);
                else res.push_back(vector<int>(temp.rbegin(), temp.rend()));
                temp.clear();
                if(q.size())                  // 当前的null不是最后一个null继续加入，否则就不再加入null了。
                    q.push(nullptr);
            }
        }

        return res;
    }
};