binary-tree-level-order-traversal-final

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:

    const std::list<TreeNode*> GetNextLevel(const std::list<TreeNode*>& lst)
    {
        std::list<TreeNode*> ret;
        for (const auto & x : lst)
        {
            if (x->left != nullptr)
                ret.push_back(x->left);
            if (x->right != nullptr)
                ret.push_back(x->right);
        }

        return ret;
    }

    vector<vector<int>> levelOrder(TreeNode* root)
    {
        vector<vector<int>> ret;

        if (root)
        {
            std::list<TreeNode*> n;
            n.push_back(root);
            ret.emplace_back(vector<int>{root->val});
            while (n.size())
            {
                vector<int> level;
                n = std::move(GetNextLevel(n));
                if (n.size())
                {
                    for (const auto & it: n)
                    {
                        level.push_back(it->val);
                    }
                    ret.push_back(level);
                }
            }
        }
        return ret;
    }
};