Maximum Level Sum of a Binary Tree

/**
 * 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:
    queue<pair<int,int>> q;
    int mxl=INT_MIN;
    int maxLevelSum(TreeNode* root)
    {
        if(root==NULL)
        {
            return root->val;
        }
        recursion(root,0);
        vector<int> v(mxl+1,0);
        while(!q.empty())
        {

            int fir=q.front().first;
            int sec=q.front().second;
            v[sec]+=fir;
            q.pop();
        }
        int mx=INT_MIN;
        int ans=-1;
        for(int i=0;i<v.size();i++)
        {
            if(v[i]>mx)
            {
                mx=v[i];
                ans=i;
            }
        }
        return ans+1;
    }
    void recursion(TreeNode* root,int level)
    {
           if(root==NULL)
           {
            return;
           }
           mxl=max(mxl,level);
           q.push({root->val,level});
           recursion(root->left,level+1);
           recursion(root->right,level+1);

    }
};