Create Randomized_QuickSelect.cpp

codes/Randomized_QuickSelect.cpp

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+// C++ implementation of randomized quickSelect 
+#include<iostream> 
+#include<climits> 
+#include<cstdlib> 
+using namespace std; 
+
+int randomPartition(int arr[], int l, int r); 
+
+// This function returns k'th smallest element in arr[l..r] using 
+// QuickSort based method. ASSUMPTION: ELEMENTS IN ARR[] ARE DISTINCT 
+int kthSmallest(int arr[], int l, int r, int k) 
+{ 
+	// If k is smaller than number of elements in array 
+	if (k > 0 && k <= r - l + 1) 
+	{ 
+		// Partition the array around a random element and 
+		// get position of pivot element in sorted array 
+		int pos = randomPartition(arr, l, r); 
+
+		// If position is same as k 
+		if (pos-l == k-1) 
+			return arr[pos]; 
+		if (pos-l > k-1) // If position is more, recur for left subarray 
+			return kthSmallest(arr, l, pos-1, k); 
+
+		// Else recur for right subarray 
+		return kthSmallest(arr, pos+1, r, k-pos+l-1); 
+	} 
+
+	// If k is more than the number of elements in the array 
+	return INT_MAX; 
+} 
+
+void swap(int *a, int *b) 
+{ 
+	int temp = *a; 
+	*a = *b; 
+	*b = temp; 
+} 
+
+// Standard partition process of QuickSort(). It considers the last 
+// element as pivot and moves all smaller element to left of it and 
+// greater elements to right. This function is used by randomPartition() 
+int partition(int arr[], int l, int r) 
+{ 
+	int x = arr[r], i = l; 
+	for (int j = l; j <= r - 1; j++) 
+	{ 
+		if (arr[j] <= x) 
+		{ 
+			swap(&arr[i], &arr[j]); 
+			i++; 
+		} 
+	} 
+	swap(&arr[i], &arr[r]); 
+	return i; 
+} 
+
+// Picks a random pivot element between l and r and partitions 
+// arr[l..r] around the randomly picked element using partition() 
+int randomPartition(int arr[], int l, int r) 
+{ 
+	int n = r-l+1; 
+	int pivot = rand() % n; 
+	swap(&arr[l + pivot], &arr[r]); 
+	return partition(arr, l, r); 
+} 
+
+// Driver program to test above methods 
+int main() 
+{ 
+	int arr[] = {12, 3, 5, 7, 4, 19, 26}; 
+	int n = sizeof(arr)/sizeof(arr[0]), k = 3; 
+	cout << "K'th smallest element is " << kthSmallest(arr, 0, n-1, k); 
+	return 0; 
+}