Add c++ fenwick tree

Signed-off-by: jparisu <[email protected]>

docs/md/intermedio/fenwick/fenwick.md

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+# Fenwick Tree
+
+```{todo}
+`Work In Progress`
+```
+
+
+## Lenguajes
+
+```{toctree}
+:maxdepth: 1
+
+fenwick_cpp
+fenwick_py
+```
+
+
+## Ejercicios
+
+| Name          | Description      | Level | URL                                            |
+|---------------|------------------|-------|------------------------------------------------|
+| fenwick       | Update & query   | 4.0   | https://open.kattis.com/problems/fenwick       |
+| supercomputer | Bitsum & biquery | 2.7   | https://open.kattis.com/problems/supercomputer |
+| downtime      |                  | 3.1   | https://open.kattis.com/problems/downtime      |

docs/md/intermedio/fenwick/fenwick_cpp.md

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+# Bucles [C++]
+
+```{contents}
+:local:
+:depth: 2
+```
+
+
+```cpp
+#include <iostream>
+#include <vector>
+using namespace std;
+
+struct FenwickTree {
+
+    // O(N)
+    // Create a Fenwick tree with n elements, all initialized to zero
+    FenwickTree(int size)
+        : n(size)
+        , tree(size)
+    {}
+
+    // O(logN)
+    // Add value to the element at index (index starts on 0)
+    void update(int index, int value) {
+        index++;
+        while (index <= n) {
+            tree[index - 1] += value;
+            index += index & -index;
+        }
+    }
+
+    // O(logN)
+    // Return the sum from 0 to index (no included) (index starts on 0)
+    int query(int index) {
+        int acc = 0;
+        while (index > 0) {
+            acc += tree[index - 1];
+            index -= index & -index;
+        }
+        return acc;
+    }
+
+    // O(logN)
+    // Return the sum from 0 to index (included) (index starts on 0)
+    int query_includes(int index) {
+        int acc = 0;
+        index++;
+        while (index > 0) {
+            acc += tree[index - 1];
+            index -= index & -index;
+        }
+        return acc;
+    }
+
+    // O(logN)
+    // Return the sum from a to b (both included)
+    int biquery(int a, int b) {
+        return query_includes(b) - query(a);
+    }
+
+    vector<int> tree;
+    int n;
+};
+```

docs/md/intermedio/fenwick/fenwick_py.md

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+# Bucles [Python]
+
+```{contents}
+:local:
+:depth: 2
+```
+
+```{todo}
+`Work In Progress`
+```

docs/md/intermedio/intermedio.md

@@ -1,6 +1,13 @@
 
 # Nivel intermedio
 
-```{todo}
-`Work In Progress`
+**1º de programación**.
+Este nivel enseña los conceptos básicos de programación y de Programación competitiva.
+
+```{toctree}
+:maxdepth: 1
+:caption: Temario
+
+entrada_salida/entrada_salida
+bucles/bucles
 ```