Segment Tree

#include <iostream>
#include <algorithm>
#include <numeric>
#include <set>
#include <queue>
#include <stack>
#include <vector>
#include <climits>
#include <string>
#include <cstdio>
#include <cmath>
#define task "TESTCODE"
using namespace std;
const int N = 1e5;
int n, Tree[4 * N + 1], a[N+1];
void read()
{
   cin >> n;
   for (int i = 1; i <= n; ++ i)
   {
       cin >> a[i];
   }
}
void BuildTree(int v, int l, int r)
{
    if (l == r)
    {
        Tree[v] = a[l];
    }
    else
    {
        int mid = (l + r)/2;
        BuildTree(v * 2, l, mid);
        BuildTree(v * 2 + 1, mid + 1, r);
        Tree[v] = Tree[2 * v] + Tree[2 * v + 1];
    }
}
void Update(int v, int TreeL, int TreeR, int pos, int new_val)
{
    if (TreeL == TreeR)
    {
        Tree[v] = new_val;
    }
    else
    {
        int mid = (TreeL + TreeR)/2;
        if (pos <= mid)
        {
            Update(v * 2, TreeL, mid, pos, new_val);
        }
        else
        {
            Update(v * 2 + 1, mid + 1, TreeR, pos, new_val);
        }
        Tree[v] = Tree[2 * v] + Tree[2 * v + 1];
    }
}
int Sum(int v, int TreeL, int TreeR, int l, int r)
{
    if (l > r)
    {
        return 0;
    }
    if (l == TreeL && r == TreeR)
    {
        return Tree[v];
    }
    int mid = (TreeL + TreeR)/2;
    return Sum(v * 2, TreeL, mid, l, min(r, mid)) + Sum(v * 2 + 1, mid + 1, TreeR, max(l, mid + 1), r);
}
void solve()
{
   BuildTree(1, 1, n);
   cout << Sum(1, 1, n, 1, 7) << '\n';
   Update(1, 1, n, 5, 69);
   cout << Sum(1,1,n,1,7);
}
int main()
{
   ios_base::sync_with_stdio(false);
   cin.tie(nullptr);
   freopen(task".INP", "r", stdin);
   //freopen(task".OUT", "w", stdout);
   read();
   solve();
}