Segment Tree Using Class

#include <bits/stdc++.h>
using namespace std;

#define P pair < int, int >
#define s second
#define f first
#define pb push_back
#define all(v)  v.begin(), v.end()
#define rep(i,a,n)  for(int i = a; i <= n; ++i)
#define lb(v, x)  lower_bound(all(v), x) - v.begin()
#define up(v, x)  upper_bound(all(v), x) - v.begin()
#define fo(i, n)   for (int i = 1; i <= n; ++i)

const int N = 3e5 + 5;
int a[N];
class seg_tree{
		public:
		int tree[N<<2];
		void build(int l, int r, int node){
		    if(l == r){
		        tree[node] = a[l];
		        return ;
		    }
		    int mid = l + r >> 1, lc = node + node, rc = 1 + lc;
		    build(l, mid, lc);
		    		 build(mid + 1, r, rc);
		    tree[node] = tree[lc] + tree[rc];
		}
		int query(int l, int r, int ql, int qr, int node){
		    if(qr < l || r < ql)
		        return 0;
		    if(ql <= l and r <= qr){
		    	return tree[node];
		    }
		    int mid = l + r >> 1, lc = node + node, rc = 1 + lc;
		    return (query(l, mid, ql, qr, lc)
		    						 + query(mid + 1, r, ql, qr, rc));
		}
		void update(int l, int r, int pos, int val, int node) {
			if(pos < l || r < pos)
				return;
			if(l == r and l == pos) {
				tree[node] = val;
				return;
			}
			int mid = l + r >> 1, lc = node + node, rc = 1 + lc;
		  update(l, mid, pos, val, lc);
		    		update(mid + 1, r, pos, val, rc);
		  tree[node] = tree[lc] + tree[rc];
		}
};
int main(){
    ios_base::sync_with_stdio(NULL);
    cin.tie(NULL);
    cout.tie(NULL);
    seg_tree obj;
   	int n, q, l, r, k;
   	cin >> n;
   	fo(i, n)
   		cin >> a[i];

    obj.build(1, n, 1);








    return 0;
}