LeetCode "Binary Tree Vertical Order"

class Solution 
{
    int maxl, minl;
    unordered_map<int, vector<int>> hm;
public:
    vector<vector<int>> verticalOrder(TreeNode* root) {
        maxl = INT_MIN;
        minl = INT_MAX;
        
        typedef pair<TreeNode*, int> Rec;
        queue<Rec> q;
        if (root)
        {
            q.push(Rec(root, 0));
        }
        while (!q.empty())
        {
            Rec curr = q.front(); q.pop();
            int l = curr.second;
            maxl = max(maxl, l);
            minl = min(minl, l);

            TreeNode *tmp = curr.first;
            hm[l].push_back(tmp->val);

            if (tmp->left)
            {
                q.push(Rec(tmp->left, l - 1));
            }
            if (tmp->right)
            {
                q.push(Rec(tmp->right, l + 1));
            }
        }

        vector<vector<int>> ret;
        for (int i = minl; i <= maxl; i++)
        {
            if (hm[i].size() == 0) continue;
            ret.push_back(hm[i]);
        }

        return ret;
    }
};