add merge sort

Author: Muskan-goyal6

.DS_Store

@@ -0,0 +1,78 @@
+
+#include "../helper.h"
+#include <iostream>
+using namespace std;
+
+void copyArr(int from[], int si, int ei, int * to) {
+    // while (si <= ei) *to++ = *si++;
+    while (si <= ei) {
+        *to = from[si];
+        ++si;
+        ++to;
+    }
+}
+
+
+void mergeSortedArray(int arr[], int si, int ei, int mid) {
+
+    int tmp_A[100];
+    int tmp_B[100];
+
+    copyArr(arr, si, mid, tmp_A);
+    copyArr(arr, mid + 1, ei, tmp_B);
+
+    int i = 0;
+    int j = 0;
+    int k = si;
+
+    //while a has elements and b has elements, I have to do something
+    int size_A = mid - si + 1;
+    int size_B = ei - (mid + 1) + 1;
+
+    while (i < size_A && j < size_B) {
+        if (tmp_A[i] < tmp_B[j]) {
+            arr[k] = tmp_A[i];
+            i++;
+            k++;
+        }
+        else {
+            arr[k++] = tmp_B[j++];
+        }
+    }
+
+    while (i < size_A) {
+        arr[k++] = tmp_A[i++];
+    }
+
+    while (j < size_B) arr[k++] = tmp_B[j++];
+
+
+}
+
+
+void mergeSort(int arr[], int si, int ei) {
+    if (si >= ei) {
+        //no elements
+        return;
+    }
+
+    int mid = (si + ei) / 2;
+    //sort the left part
+    mergeSort(arr, si, mid);
+    mergeSort(arr, mid + 1, ei);
+
+    mergeSortedArray(arr, si, ei, mid);
+
+}
+
+
+int main() {
+    int arr[100];
+    int n;
+    cin >> n;
+    inputArr(arr, n);
+
+    mergeSort(arr, 0, n - 1);
+
+    printArr(arr, n);
+}

Mersort_In_C++/mergeSort.cpp

@@ -0,0 +1,13 @@
+Merge Sort is a Divide and Conquer algorithm. It divides input array in 
+two halves, calls itself for the two halves and then merges the two sorted halves.
+Algorithm:
+MergeSort(arr[], l,  r)
+If r > l
+1. Find the middle point to divide the array into two halves:  
+             middle m = (l+r)/2
+2. Call mergeSort for first half:   
+             Call mergeSort(arr, l, m)
+3. Call mergeSort for second half:
+             Call mergeSort(arr, m+1, r)
+4. Merge the two halves sorted in step 2 and 3:
+             Call merge(arr, l, m, r)