Update fenwick_tree.cpp

Author: scanof

code/data-structures/fenwick_tree.cpp

@@ -1,13 +1,13 @@
-struct fwtree{ // 1-indexed
+struct fwtree{ // 0-indexed
   int n;  vi bit;
   fwtree(int n): n(n), bit(n+1){}
   int rsq(int r){
     int sum = 0;
-    for(; r; r -= r & -r) sum += bit[r];
+    for(r+=1; r; r -= r & -r) sum += bit[r];
     return sum;
   }
-  int rsq(int l, int r){return rsq(r) - rsq(l-1);}
+  int rsq(int l, int r){return rsq(r) - (l==0 ? 0 : rsq(l-1));}
   void upd(int r, int v){
-    for(; r <= n; r += r & -r) bit[r] += v;
+    for(r+=1; r <= n; r += r & -r) bit[r] += v;
   }
 };