Added mergesort.cpp algo (#20)

Co-authored-by: Aniket <[email protected]>

Algos/merge-sort.cpp

@@ -0,0 +1,62 @@
+#include <iostream>
+using namespace std;
+
+void merge(int *,int,int,int);
+void merge(int *arr,int low,int high,int mid){
+
+int i,j,k,a[100];
+i=low;
+j=mid+1;
+k=low;
+while (i <= mid && j <= high) {
+        if (arr[i] < arr[j]) {
+            a[k] = arr[i];
+            k++;
+            i++;
+        }
+        else  {
+            a[k] = arr[j];
+            k++;
+            j++;
+        }
+    }
+    while (i <= mid) {
+        a[k] = arr[i];
+        k++;
+        i++;
+    }
+    while (j <= high) {
+        a[k] = arr[j];
+        k++;
+        j++;
+    }
+    for (i = low; i < k; i++)  {
+        arr[i] = a[i];
+    }
+}
+void merge_sort(int *arr, int low, int high)
+{
+    int mid;
+    if (low < high){
+        mid=(low+high)/2;
+        merge_sort(arr,low,mid);
+        merge_sort(arr,mid+1,high);
+        merge(arr,low,high,mid);
+    }
+}
+
+
+int main(){
+    int n,array[100];
+    cout<<"enter no to be sorted:";
+    cin>>n;
+    cout<<"enter numbers for sorting";
+    for(int i=0;i<n;i++){
+        cin>>array[i];
+    }
+    merge_sort(array,0,n-1);
+    cout<<"sorted array\n";
+    for(int i=0;i<n;i++){
+        cout<<array[i]<<'\t';
+    }
+}