added mergesort.

contributors.md

@@ -8,4 +8,7 @@
 6. [Ujjval Patel](https://github.com/Ujjval-Patel)
 7. [Shaguna Awasthi](https://github.com/Shagunaawasthi)
 8. [Joy Banerjee](https://github.com/joybanerjee08)
-9. [Kartikey Tripathi](https://github.com/kartikeytripathi)
+9. [Nikhil Arora](https://github.com/nikhilarora068)
+10. [Ambika](https://github.com/Ambika55)
+11. [Kartikey Tripathi](https://github.com/kartikeytripathi)
+12. [Ananya Keshari](https://github.com/ana301)

sorting/mergesort.cpp

@@ -0,0 +1,63 @@
+
+
+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; // Initial index of first subarray 
+    j = 0; // Initial index of second subarray 
+    k = l; // Initial index of merged subarray 
+    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++; 
+    } 
+} 
+{ 
+    if (l < r) 
+    { 
+        // Same as (l+r)/2, but avoids overflow for 
+        // large l and h 
+        int m = l+(r-l)/2; 
+
+        // Sort first and second halves 
+        mergeSort(arr, l, m); 
+        mergeSort(arr, m+1, r); 
+
+        merge(arr, l, m, r); 
+    } 
+}