add quickselect.cpp file

Algorithms/Selection_Algorithms/QuickSelect/quickselect.cpp

@@ -0,0 +1,65 @@
+// CPP program for implementation of QuickSelect 
+#include <bits/stdc++.h> 
+using namespace std; 
+
+// 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 
+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; 
+} 
+
+// This function returns k'th smallest 
+// element in arr[l..r] using QuickSort 
+// based method. ASSUMPTION: ALL 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 last 
+		// element and get position of pivot 
+		// element in sorted array 
+		int index = partition(arr, l, r); 
+
+		// If position is same as k 
+		if (index - l == k - 1) 
+			return arr[index]; 
+
+		// If position is more, recur 
+		// for left subarray 
+		if (index - l > k - 1) 
+			return kthSmallest(arr, l, index - 1, k); 
+
+		// Else recur for right subarray 
+		return kthSmallest(arr, index + 1, r, 
+							k - index + l - 1); 
+	} 
+
+	// If k is more than number of 
+	// elements in array 
+	return INT_MAX; 
+} 
+
+// Driver program to test above methods 
+int main() 
+{ 
+	int arr[] = { 10, 4, 5, 8, 6, 11, 26 }; 
+	int n = sizeof(arr) / sizeof(arr[0]); 
+	int k = 3; 
+	cout << "K-th smallest element is "
+		<< kthSmallest(arr, 0, n - 1, k); 
+	return 0; 
+}