我的代码拿了40分,看到题解的修改操作都比我复杂(?)是不是我理解错了

#include <bits/stdc++.h>
#define int long long
#define maxn 100005
using namespace std;
int n, m, a[maxn], s[maxn], ansn[maxn << 2], tag[maxn << 2];
int ls(int p) {
	return p << 1;
}
int rs(int p) {
	return p << 1 | 1;
}
void push_up(int p) {
	ansn[p] = ansn[ls(p)] + ansn[rs(p)];
}
void build(int l, int r, int p) {
	int mid = (l + r) >> 1;
	if(l == r) {
		ansn[p] = s[l];
		return;
	}
	build(l, mid, ls(p));
	build(mid + 1, r, rs(p));
	push_up(p);
}
void f(int p, int l, int r, int k) {
	ansn[p] += k * (r - l + 1);
	tag[p] += k;
}
void push_down(int p, int l, int r) {
	int mid = (l + r) >> 1;
	f(ls(p), l, mid, tag[p]);
	f(rs(p), mid + 1, r, tag[p]);
	tag[p] = 0;
}
void update(int nl, int nr, int l, int r, int p, int k) {
	int mid = (l + r) >> 1;
	if(nl <= l && r <= nr) {
		ansn[p] += k * (r - l + 1);
		tag[p] += k;
		return;
	}
	push_down(p, l, r);
	if(nl <= mid) update(nl, nr, l, mid, ls(p), k);
	if(nr > mid) update(nl, nr, mid + 1, r, rs(p), k);
	push_up(p);
}
int query(int nl, int nr, int l, int r, int p) {
	int mid = (l + r) >> 1, res = 0;
	if(nl <= l && r <= nr) return ansn[p];
	push_down(p, l, r);
	if(nl <= mid) res += query(nl, nr, l, mid, ls(p));
	if(nr > mid) res += query(nl, nr, mid + 1, r, rs(p));
	return res;
}
signed main() {
	scanf("%lld%lld", &n, &m);
	for(int i = 1;i <= n;i++) {
		scanf("%lld", &a[i]);
		s[i] = a[i] + s[i - 1];
	}
	build(1, n, 1);
	for(int i = 1;i <= m;i++) {
		char mps[205];
		int x, y;
		cin >> mps;
		if(mps[0] == 'M') {
			scanf("%lld%lld", &x, &y);
			update(x, n, 1, n, 1, y - a[x]);
		}
		else {
			scanf("%lld", &x);
			printf("%lld\n", query(1, x, 1, n, 1));
		}
	}
}