把二叉树打印成多行

从上到下按层打印二叉树，同一层结点从左至右输出。每一层输出一行。

开始打印任意一层前记录list大小，以区分各行

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
public:
        vector<vector<int> > Print(TreeNode* pRoot) {
            if (pRoot == nullptr)
            {
                return {};
            }
            else
            {
                vector<vector<int>> result;
                list<TreeNode*> l{pRoot};
                int size;
                while (size = l.size())
                {
                    vector<int> row;
                    row.reserve(size);
                    for (; size > 0; size--)
                    {
                        TreeNode *n = l.front();
                        l.pop_front();
                        row.push_back(n->val);
                        if (n->left != nullptr)
                        {
                            l.push_back(n->left);
                        }
                        if (n->right != nullptr)
                        {
                            l.push_back(n->right);
                        }
                    }
                    result.push_back(row);
                }
                return result;
            }
        }
};