add range sum to BIT

atcoder-live · atcoder-live · commit 875d2bee61c3 · 2020-04-22T00:39:44.000+09:00
diff --git a/bit.cpp b/bit.cpp
@@ -17,4 +17,7 @@ struct BIT {
     }
     return x;
   }
+  T sum(int l, int r) {
+    return sum(r-1) - sum(l-1);
+  }
 };
