Added BIT

Range queries/bit.cpp

@@ -0,0 +1,66 @@
+// Binary Indexed Tree.
+#include<bits/stdc++.h>
+using namespace std;
+
+class Bit{
+  int n;
+  vector<int> bit;
+  inline int offset(int x){ return (x & (-x)); }
+
+	public:
+
+		Bit(vector<int>& arr){
+			n = arr.size();
+			bit.assign(n+1,0);
+			for(int i=0; i<n; ++i){
+				update(i,arr[i]);
+			}
+		}
+		Bit(int x){
+			n = x;
+			bit.assign(n+1,0);
+		}
+
+  void update(int id, int val){
+    // Add val at id
+    id++;
+    while( id <= n ){
+      bit[id] += val;
+      id += offset(id);
+    }
+  }
+
+  int sum(int id){
+    // Get prefix sum upto id.
+    id++;
+    int res = 0;
+    while( id > 0 ){
+      res += bit[id];
+      id -= offset(id);
+    }
+		return res;
+  }
+
+  int sum_range(int l, int r){
+    return sum(r) - sum(l-1);
+  }
+};
+
+
+
+int main(){
+
+	int n = 5;
+	vector<int> arr = {1,2,3,4,5};
+	Bit x(arr);
+
+	assert(x.sum_range(0,0) == 1);
+	assert(x.sum_range(0,1) == 3);
+	assert(x.sum_range(0,2) == 6);
+	x.update(0,6);
+	assert(x.sum_range(0,0) == 6);
+	assert(x.sum_range(0,1) == 8);
+	assert(x.sum_range(0,2) == 11);
+
+
+}