added shellsort algorithm

ShellSort/README.md

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+## Shell Short
+
+Shellsort is an in-place comparison sort. It can either be seen as a generalization of sorting by exchange (bubble sort) or sorting by insertion (insertion sort). The method starts by sorting elements far apart from each other and progressively reducing the gap between them. Starting with far apart elements can move some out-of-place elements into position faster than a simple nearest neighbor exchange. Worst case time complexity is O(n2) and best case complexity is O(nlog(n)).

ShellSort/shellsort.c

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+/*
+ * C Program to sort an array using Shell Sort technique
+ */
+#include <stdio.h>
+void shellsort(int arr[], int num)
+{
+    int i, j, k, tmp;
+    for (i = num / 2; i > 0; i = i / 2)
+    {
+        for (j = i; j < num; j++)
+        {
+            for(k = j - i; k >= 0; k = k - i)
+            {
+                if (arr[k+i] >= arr[k])
+                    break;
+                else
+                {
+                    tmp = arr[k];
+                    arr[k] = arr[k+i];
+                    arr[k+i] = tmp;
+                }
+            }
+        }
+    }
+}
+int main()
+{
+    int arr[30];
+    int k,  num;
+    printf("Enter total no. of elements : ");
+    scanf("%d", &num);
+    printf("\nEnter %d numbers: ", num);
+
+    for (k =  0 ; k < num; k++)
+    {
+        scanf("%d", &arr[k]);
+    }
+    shellsort(arr, num);
+    printf("\n Sorted array is: ");
+    for (k = 0; k < num; k++)
+        printf("%d ", arr[k]);
+    return 0;
+}