update merge-sort to reflect API update

sorting.cpp

 #include "sorting.h"
 using namespace std ;
 //random
-void merge(int arr[], int l, int m, int r) 
-{ 
-    int i, j, k; 
-    int n1 = m - l + 1; 
-    int n2 =  r - m; 
-    int L[n1], R[n2]; 
-    for (i = 0; i < n1; i++) 
-        L[i] = arr[l + i]; 
-    for (j = 0; j < n2; j++) 
-        R[j] = arr[m + 1+ j]; 
-    i = 0;  
-    j = 0; 
-    k = l;  
-    while (i < n1 && j < n2) 
-    { 
-        if (L[i] <= R[j]) 
-        { 
-            arr[k] = L[i]; 
-            i++; 
-        } 
+void merge(int arr[], int l, int m, int r)
+{
+    int i, j, k;
+    int n1 = m - l + 1;
+    int n2 =  r - m;
+    int L[n1], R[n2];
+    for (i = 0; i < n1; i++)
+        L[i] = arr[l + i];
+    for (j = 0; j < n2; j++)
+        R[j] = arr[m + 1+ j];
+    i = 0;
+    j = 0;
+    k = l;
+    while (i < n1 && j < n2)
+    {
+        if (L[i] <= R[j])
+        {
+            arr[k] = L[i];
+            i++;
+        }
         else
-        { 
-            arr[k] = R[j]; 
-            j++; 
-        } 
-        k++; 
-    } 
-    while (i < n1) 
-    { 
-        arr[k] = L[i]; 
-        i++; 
-        k++; 
-    } 
-    while (j < n2) 
-    { 
-        arr[k] = R[j]; 
-        j++; 
-        k++; 
-    } 
-} 
+        {
+            arr[k] = R[j];
+            j++;
+        }
+        k++;
+    }
+    while (i < n1)
+    {
+        arr[k] = L[i];
+        i++;
+        k++;
+    }
+    while (j < n2)
+    {
+        arr[k] = R[j];
+        j++;
+        k++;
+    }
+}
 
-void mergeSort(int arr[], int l, int r) 
-{ 
-    if (l < r) 
-    { 
-        int m = l+(r-l)/2;  
-        mergeSort(arr, l, m); 
-        mergeSort(arr, m+1, r); 
-  
-        merge(arr, l, m, r); 
-    } 
-} 
+void mergeSort(int arr[], int l, int r)
+{
+    if (l < r)
+    {
+        int m = l+(r-l)/2;
+        mergeSort(arr, l, m);
+        mergeSort(arr, m+1, r);
+
+        merge(arr, l, m, r);
+    }
+}
 
-vector<int> sort_custom(vector<int> data)
+vector<int> sort_custom(vector<int> data, int startidx, int endidx)
 {
 	//vector<int> arr(data.begin(), data.end()) ;
     int n = data.size() ;
@@ -63,7 +63,7 @@ vector<int> sort_custom(vector<int> data)
     {
         arr[i] = data[i] ;
     }
-    mergeSort(arr,0,n-1) ;
-    vector<int> res(arr, arr+n) ;
+    mergeSort(arr, startidx, endidx - 1) ;
+    vector<int> res(arr, arr + n) ;
     return res ;
-}
+}
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