Merge Sort (#231)

* Add @imkaka as a contributor * day4 * Merge Sort
CodeToExpress · Feb 7, 2019 · aaff040 · aaff040
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diff --git a/day34/C++/mergeSort.cpp b/day34/C++/mergeSort.cpp
@@ -0,0 +1,94 @@
+/*
+* @author : imkaka
+* @date   : 5/2/2019
+*/
+
+#include<iostream>
+#include<vector>
+
+using namespace std;
+
+// Merges two subarrays of arr[].
+void merge(vector<int>& arr, int l, int m, int r)
+{
+    int i, j, k;
+    int n1 = m - l + 1;
+    int n2 =  r - m;
+
+    //create temp arrays
+    int L[n1], R[n2];
+
+    //Copy data to temp arrays L[] and R[]
+    for (i = 0; i < n1; i++)
+        L[i] = arr[l + i];
+    for (j = 0; j < n2; j++)
+        R[j] = arr[m + 1+ j];
+
+    // Merge the temp arrays back
+    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++;
+    }
+
+    // Copy the remaining elements of L[]
+    while (i < n1)
+    {
+        arr[k] = L[i];
+        i++;
+        k++;
+    }
+
+    //Copy the remaining elements of R[]
+    while (j < n2)
+    {
+        arr[k] = R[j];
+        j++;
+        k++;
+    }
+}
+
+void mergeSort(vector<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 print(const vector<int>& arr){
+    for(const int x : arr){
+        cout << x << " ";
+    }
+
+    cout << endl;
+}
+int main(){
+    vector<int> arr = {12, 34, 1, -5, 10, 43, 23, 100, 100000, 0};
+
+    cout << "Before Sorting : ";
+    print(arr);
+
+    mergeSort(arr, 0, arr.size()-1);
+
+    cout << endl << "After Sorting : ";
+    print(arr);
+    return 0;
+}
diff --git a/day34/README.md b/day34/README.md
@@ -25,4 +25,103 @@ output: [1, 2, 3, 4, 5, 6, 7, 8, 9]
 
 ```js
 to be added
-```
+```
+
+### [C++ Implementation](./C++/mergeSort.cpp)
+
+```cpp
+/*
+* @author : imkaka
+* @date   : 5/2/2019
+*/
+
+#include<iostream>
+#include<vector>
+
+using namespace std;
+
+// Merges two subarrays of arr[].
+void merge(vector<int>& arr, int l, int m, int r)
+{
+    int i, j, k;
+    int n1 = m - l + 1;
+    int n2 =  r - m;
+
+    //create temp arrays
+    int L[n1], R[n2];
+
+    //Copy data to temp arrays L[] and R[]
+    for (i = 0; i < n1; i++)
+        L[i] = arr[l + i];
+    for (j = 0; j < n2; j++)
+        R[j] = arr[m + 1+ j];
+
+    // Merge the temp arrays back
+    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++;
+    }
+
+    // Copy the remaining elements of L[]
+    while (i < n1)
+    {
+        arr[k] = L[i];
+        i++;
+        k++;
+    }
+
+    //Copy the remaining elements of R[]
+    while (j < n2)
+    {
+        arr[k] = R[j];
+        j++;
+        k++;
+    }
+}
+
+void mergeSort(vector<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 print(const vector<int>& arr){
+    for(const int x : arr){
+        cout << x << " ";
+    }
+
+    cout << endl;
+}
+int main(){
+    vector<int> arr = {12, 34, 1, -5, 10, 43, 23, 100, 100000, 0};
+
+    cout << "Before Sorting : ";
+    print(arr);
+
+    mergeSort(arr, 0, arr.size()-1);
+
+    cout << endl << "After Sorting : ";
+    print(arr);
+    return 0;
+}
+```