Added FenwickTree For Range Query

Range queries/FenwickTree.cpp

@@ -0,0 +1,56 @@
+#include<iostream>
+using namespace std;
+
+/**
+ * ` lowbit(x) ` aims to find the last 1 in binary of a positive number
+ * twos complement works good on this
+ * also using ` x - (x & (x - 1)) `
+ */
+#define lowbit(x) (x & (-x) )
+
+const int maxn = 1e5 + 7;
+int tree[maxn] = {0},
+                 range;             // segement of [1...range], notice it must be less than `maxn`
+
+void update(int x, int c) {
+    while(x <= range) {
+        tree[x] += c;
+        x += lowbit(x);
+    }
+}
+int query(int x) {
+    int ans = 0;
+    while(x) {
+        ans += tree[x];
+        x -= lowbit(x);
+    }
+    return ans;
+}
+int query_segement(int l, int r) {
+    return query(r) - query(l - 1);
+}
+
+int main() {
+    cin >> range;
+    for(int i = 1; i <= range; i++) {
+        int num;
+        cin >> num;
+        update(i, num);
+    }
+    int q;
+    cin >> q;
+    while(q--) {
+        int op;
+        cin >> op;
+        if(op == 0) {
+            int l, r;
+            cin >> l >> r;
+            cout << query_segement(l, r) << endl;
+        } else {
+            int x, c;
+            cin >> x >> c;
+            update(x, c);
+        }
+    }
+    return 0;
+}