輸出二叉樹的層序遍歷序列
如[2,6,8,null,null,10,12]，輸出[[2],[6,8],[10,12]]
另：若要逆序輸出，即[[10,12],[6,8],[2]]，則只需return前reverse(result.begin(),result.end());
Binary Tree Level Order Traversal
藉助二維vector跟num，依次插入各層

/**
 * 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>> levelOrder(TreeNode* root) {
        vector<vector<int>> result;
        level(result,root,1);
        //reverse(result.begin(),result.end());

        return result;
    }

    void level(vector<vector<int>> &result,TreeNode *root,int
 
 num)
    {
        if(root==NULL)
            return;
        if(num>result.size())
            result.push_back(vector<int>());

        result[num-1].push_back(root->val);
        level(result,root->left,num+1);
        level(result,root->right,num+1);
    }
};