updated merge-sort (#1086)

* updated merge-sort

* updated merge-sort

* updated merge-sort

* Update

Algorithms-Merge-Sort.md

@@ -11,38 +11,127 @@ To learn about Merge Sort, a basic knowledge about [Recursion](http://programmer
 - Merge the sorted halves.
 
 The biggest advantage of using Merge sort is that the [time complexity](https://www.youtube.com/watch?v=V42FBiohc6c&list=PL2_aWCzGMAwI9HK8YPVBjElbLbI3ufctn) is only n*log(n) to sort an entire Array. It is a lot better than n^2 running time of bubble sort or insertion sort.
-Before we write the JavaScript code, let us understand how merge sort works with the help of a diagram.
+Before we write code, let us understand how merge sort works with the help of a diagram.
 
-![alt tag] (https://i67.tinypic.com/2ahe49y.png)
+![Merge Sort](https://i.imgur.com/ac98ccu.png)
 
 - Initially we have an array of 6 unsorted integers Arr(5, 8, 3, 9, 1, 2)
 - We split the array into two halves Arr1 = (5, 8, 3) and  Arr2 = (9, 1, 2).
 -  Again, we divide them into two halves: Arr3 = (5, 8) and Arr4 = (3) and  Arr5 = (9, 1) and Arr6 = (2)
 -  Again, we divide them into two halves: Arr7 = (5), Arr8 =  (8),  Arr9 = (9),  Arr10 = (1) and Arr6 = (2)
--  We will now compare the elements in these sub arrays in order to merge them. 
+-  We will now compare the elements in these sub arrays in order to merge them.
+
+## Implementation
+
+### C++ Implementation
+
+```c++
+void merge(int array[], int left, int mid, int right)
+{
+    int i, j, k;
+
+    // Size of left sublist
+    int size_left = mid - left + 1;
+
+    // Size of right sublist
+    int size_right =  right - mid;
+
+    /* create temp arrays */
+    int Left[size_left], Right[size_right];
+
+    /* Copy data to temp arrays L[] and R[] */
+    for(i = 0; i < size_left; i++)
+    {
+        Left[i] = array[left+i];
+    }
+
+    for(j = 0; j < size_right; j++)
+    {
+        Right[j] = array[mid+1+j];
+    }
+
+    // Merge the temp arrays back into arr[left..right]
+    i = 0; // Initial index of left subarray
+    j = 0; // Initial index of right subarray
+    k = left; // Initial index of merged subarray
+
+    while (i < size_left && j < size_right)
+    {
+        if (Left[i] <= Right[j])
+        {
+            array[k] = Left[i];
+            i++;
+        }
+        else
+        {
+            array[k] = Right[j];
+            j++;
+        }
+        k++;
+    }
+
+    // Copy the remaining elements of Left[]
+    while (i < size_left)
+    {
+        array[k] = Left[i];
+        i++;
+        k++;
+    }
+
+    // Copy the rest elements of R[]
+    while (j < size_right)
+    {
+        array[k] = Right[j];
+        j++;
+        k++;
+    }
+}
+```
+
+```c++
+void mergeSort(int array[], int left, int right)
+{
+    if(left < right)
+    {
+        int mid = (left+right)/2;
+
+        // Sort first and second halves
+        mergeSort(array, left, mid);
+        mergeSort(array, mid+1, right);
+
+        // Finally merge them
+        merge(array, left, mid, right);
+    }
+}
+```
+
+:rocket: [Run Code](https://repl.it/CYVc/1)
+
+### Javascript Implementation
 
 Let's write MergeSort in JavaScript:
 
 ```javascript
-function mergeSort (arr) {    
+function mergeSort (arr) {
   if (arr.length < 2) return arr;
   var mid = Math.floor(arr.length /2);
   var subLeft = mergeSort(arr.slice(0,mid));
   var subRight = mergeSort(arr.slice(mid));
   return merge(subLeft, subRight);
-  }
-
+}
 ```
-First we check the length of the array. If it is 1 then we simply return the array. This would be our base case. Else, we will find out the middle value and divide the array into two halves. We will now sort both of the halves with recursive calls to MergeSort function. 
+
+First we check the length of the array. If it is 1 then we simply return the array. This would be our base case. Else, we will find out the middle value and divide the array into two halves. We will now sort both of the halves with recursive calls to MergeSort function.
+
 ```javascript
 function merge (a,b) {
     var result = [];
     while (a.length >0 && b.length >0)
         result.push(a[0] < b[0]? a.shift() : b.shift());
     return result.concat(a.length? a : b);
-    }
-
+}
 ```
+
 When we merge the two halfs, we store the result in an auxilliary array. We will compare the starting element of left array to the starting element of right array. Whichever is lesser will be pushed into the results array and we will remove it from there respective arrays using [shift() operator](https://github.com/FreeCodeCamp/FreeCodeCamp/wiki/js-Array-prototype-shift). If we still end up with values in either of left or right array, we would simply concatenate it in the end of the result.
 
 Here is the sorted result:
@@ -54,4 +143,6 @@ console.log(mergeSort(test));
 >> [1, 3, 3, 5, 6, 7, 15]
 ```
 
-If you still have problem in understanding MergeSort, a [video explanation] (https://www.youtube.com/watch?v=TzeBrDU-JaY) will make it even more clear. 
+:rocket: [Run Code](https://repl.it/CYVd)
+
+If you still have problem in understanding MergeSort, a [video explanation] (https://www.youtube.com/watch?v=TzeBrDU-JaY) will make it even more clear.