shell_sort.cpp Adds Shell Sort Algorithm

Closes #270

shell_sort/Cpp/shell_sort.cpp

@@ -0,0 +1,62 @@
+#include <iostream>
+
+using std::cout;
+
+void insertion_sort(int a[], int a_size) {
+    int i = 0;
+    while (i < a_size) {
+        int j = i;
+        while (j > 0 && a[j-1] > a[j]) {
+            // swap
+            int temp = a[j-1];
+            a[j-1] = a[j];
+            a[j] = temp;
+            j--;
+        }
+        i++;
+    }
+
+    cout << "After insertion sort: ";
+    for (int i = 0; i < a_size; i++) {
+        cout << a[i] << " ";
+    }
+    cout << std:: endl;
+}
+
+void shell_sort(int a[], int a_size) {
+    int gap = 1;
+    while (gap < a_size/3) {
+        gap = gap * 3 + 1; //calcutale biggest gap to start with.
+    }
+
+    while (gap > 0) {
+        for (int i = 0; i < a_size - gap; i++) {
+            if (a[i] > a[i + gap]) {
+                // swap
+                int temp = a[i];
+                a[i] = a[i +  gap];
+                a[i + gap] = temp;
+            }
+        }
+        gap = (gap - 1) / 3;
+    }
+
+    cout << "After shell sort: ";
+    for (int i = 0; i < a_size; i++) {
+        cout << a[i] << " ";
+    }
+    cout << std:: endl;
+
+    insertion_sort(a, a_size);
+}
+
+int main(void) {
+    int a[8] = {3, 7, 4, 9, 5, 2, 6, 1};
+    int a_size = sizeof(a) / sizeof(a[0]);
+
+    shell_sort(a, a_size);
+
+    for (int i = 0; i < a_size; i++) {
+        cout << a[i] << " ";
+    }
+}