added randomized quicksort algorithm

this quicksort algorithm works taking a random element from the subarray as the pivot element

Author: dipanmandal

Diff for: randomized_quickSort.cpp

@@ -0,0 +1,63 @@
+#include<bits/stdc++.h>
+using namespace std;
+
+int random(int lower, int upper){ //function to generate a random number in a given range
+    
+    int num = (rand() %(upper - lower + 1)) + lower;
+    return num;
+}
+
+void printArray(int* arr, int size){
+    for(int i = 0 ; i < size; i++) cout<<arr[i]<<" ";
+    cout<<endl;
+}
+
+int partition(int* arr, int start , int end){
+
+    int index = random(start, end); //select a random index in the subarray
+    swap(arr[start], arr[index]); //swap that random index with the first element of that subarray
+
+    //continue with the normal quicksort partition subroutine
+
+    int pivot = arr[start];
+    int i = start;
+
+    for(int j = start +1; j<= end; j++){
+        if(arr[j]<= pivot){
+            i++;
+            if (i != j) swap(arr[i], arr[j]);
+        }
+    }
+    swap(arr[start], arr[i]);
+
+    return i;
+
+}
+
+void randomized_quickSort(int* arr, int p, int r){
+    if (p < r){ //this line is important as it controls the recursion calls
+        int q = partition(arr, p , r);
+        randomized_quickSort(arr, p, q-1);
+        randomized_quickSort(arr, q+1, r);
+    }
+}
+
+int main(){
+    int n;
+    cout<<"enter the number of elements in the array: "<<endl;
+    cin>>n;
+
+    int arr[n];
+
+    cout<<"enter the elements: "<<endl;
+
+    for(int i = 0; i < n; i++)cin>>arr[i];
+
+    cout<<"the unsorted array is: "<<endl;
+    printArray(arr,n);
+
+    randomized_quickSort(arr, 0, n-1); 
+
+    cout<<"the sorted array is: "<<endl;
+   printArray(arr, n);
+}