update merge-sort to reflect API update

sorting.cpp

-#include "sorting.h"
+#include"sorting.h"
 
-vector<int> merge(vector<int> left, vector<int> right)
+
+void merge(vector<int>& v, int first, int mid, int last)
 {
-   vector<int> result;
-   while ((int)left.size() > 0 || (int)right.size() > 0) {
-      if ((int)left.size() > 0 && (int)right.size() > 0) {
-         if ((int)left.front() <= (int)right.front()) {
-            result.push_back((int)left.front());
-            left.erase(left.begin());
-         } 
-   else {
-            result.push_back((int)right.front());
-            right.erase(right.begin());
-         }
-      }  else if ((int)left.size() > 0) {
-            for (int i = 0; i < (int)left.size(); i++)
-               result.push_back(left[i]);
-            break;
-      }  else if ((int)right.size() > 0) {
-            for (int i = 0; i < (int)right.size(); i++)
-               result.push_back(right[i]);
-            break;
+  // temporary vector to merge the sorted sublists
+  vector<int> tempVector;
+  int indexA, indexB, indexV;
+  
+  // set indexA to scan sublistA (index range [first,mid)
+  // and indexB to scan sublistB (index range [mid, last)
+  indexA = first;
+  indexB = mid;
+  
+  // while both sublists are not exhausted, compare v[indexA] and
+  // v[indexB]copy the smaller to vector temp using push_back()
+  while (indexA < mid && indexB < last)
+    if (v[indexA] < v[indexB])
+      {
+        tempVector.push_back(v[indexA]);    // copy element to temp
+        indexA++;                           // increment indexA
+      }
+    else
+      {
+        tempVector.push_back(v[indexB]);    // copy element to temp
+        indexB++;                           // increment indexB
       }
-   }
-   return result;
+  
+  // copy the tail of the sublist that is not exhausted
+  while (indexA < mid)
+    {
+      tempVector.push_back(v[indexA]);
+      indexA++;
+    }
+  
+  while (indexB < last)
+    {
+      tempVector.push_back(v[indexB]);
+      indexB++;
+    }
+  
+  // copy vector tempVector using indexV to vector v using indexA
+  // which is initially set to first
+  indexA = first;
+  
+  // copy elements from temporary vector to original list
+  for (indexV = 0; indexV < tempVector.size(); indexV++)
+    {
+      v[indexA] = tempVector [indexV];
+      indexA++;
+    }
 }
 
-vector<int> sort_custom(vector<int> m)
+vector<int> sort_custom (vector<int>
+ v, int first, int last)
 {
-   if (m.size() <= 1)
-      return m;
- 
-   vector<int> left, right, result;
-   int mid = ((int)m.size()+ 1) / 2;
- 
-   for (int i = 0; i < mid; i++) {
-      left.push_back(m[i]);
-   }
-
-   for (int i = mid; i < (int)m.size(); i++) {
-      right.push_back(m[i]);
-   }
- 
-   left = sort_custom(left);
-   right = sort_custom(right);
-   result = merge(left, right);
- 
-   return result;
-}
-
+  // if the sublist has more than 1 element continue
+  if (first + 1 < last)
+    {
+      // for sublists of size 2 or more, call mergeSort()
+      // for the left and right sublists and then
+      // merge the sorted sublists using merge()
+      int midpt = (last + first) / 2;
+      
+      sort_custom(v, first, midpt);
+      sort_custom(v, midpt, last);
+      merge(v, first, midpt, last);
+    }
+ return v;
+}
\ No newline at end of file