1. • for (int i = 0; i < n; i++) {
2. • ll rightMax = query(prefixTree, 1, 0, _n — 1, i + 1, ng[i] — 1) — prefixSum[i];
3. • ll leftMax = query(suffixTree, 1, 0, _n — 1, pg[i] + 1, i — 1) — suffixSum[i];
4. • if (max(leftMax, rightMax) > 0) {
5. • flag = false;
6. • break;
7. • }
8. • }

why they are selecting the lower index as i + 1 but not i. According to me it was something like

ll rightMax = query(prefixTree, 1, 0, _n — 1, i, ng[i] — 1) — prefixSum[i-1];**