Merge-Sort Tree

struct MergeSort_Tree {
   int n;
   vector<vector<int>> t;
   vector<int> a;
   MergeSort_Tree(int _n, vector<int> _a) {
     n = _n;
     t.resize(5 * n, vector<int>());
     for(int i = 0; i < _a.size(); i++) {
      a.push_back(_a[i]);
     }
   } 
   vector<int> merge(vector<int> p, vector<int> q) {
     vector<int> r;
     int i = 0, j = 0;
     while(i < p.size() && j < q.size()) {
      if(p[i] < q[j]) {
        r.push_back(p[i++]);
      } else {
        r.push_back(q[j++]);
      }
     }
     while(i < p.size()) {
      r.push_back(p[i++]);
     }
     while(j < q.size()) {
      r.push_back(q[j++]);
     }
     return r;
   }
   void build(int curr, int l, int r) {
     if(l == r) {
      t[curr].push_back(a[l]);
      return;
     }
     int mid = (l + r) / 2;
     build(2 * curr + 1, l, mid);
     build(2 * curr + 2, mid + 1, r);
     t[curr] = merge(t[2 * curr + 1], t[2 * curr + 2]);
   }
   int query(int curr, int l, int r, int x, int y, int k) {
     if(r < x || l > y) {
      return 0;
     }
     if(x <= l && r <= y) {
      // for smaller use lower_bound and for smaller equal use upper_bound
      return lower_bound(t[curr].begin(), t[curr].end(), k) - t[curr].begin(); 
     }
     int mid = (l + r) / 2;
     return query(2 * curr + 1, l, mid, x, y, k) + query(2 * curr + 2, mid + 1, r, x, y, k);
   }

   int query(int l, int r, int k) {
    return query(0, 0, n - 1, l, r , k);
   }
};