Segment_Tree_Point_Update

const int MAX = 1e5 + 10;
int a[MAX], tree[MAX * 4];

void build(int at, int L, int R) {
    if (L == R) {
        tree[at] = a[L];
        return;
    }
    int mid = (L + R) / 2;
    int left = 2 * at;
    int right = 2 * at + 1;
    build(left, L, mid);
    build( right, mid + 1, R);
    tree[at] = tree[left] + tree[right];
}
//call it using build(1, 0, arraySize-1);


void update(int at, int L, int R, int pos, int val) {
    if (pos > R || pos < L) return;
    if (L == R) {
        tree[at] = val;
        return;
    }
    int mid = (L + R) / 2;
    int left = at * 2;
    int right = at * 2 + 1;
    update(left, L, mid, pos, val);
    update(right, mid + 1, R, pos, val);
    tree[at] = tree[left] + tree[right];
}
//call it using update(1,0,arraySize-1,l,r,val);

int query(int at, int L, int R, int l, int r) {
    if (l > R || r < L) return 0;
    if (L >= l && R <= r) {
        return tree[at];
    }
    int mid = (L + R) / 2;
    int left = at * 2;
    int right = at * 2 + 1;
    int x = query(left, L, mid, l, r);
    int y = query(right, mid + 1, R, l, r);
    return x + y;
}
//call it using query(1,0,arraySize-1,l,r);