From 46ad5baa19f23866e2bb4a17fe1f7f65f765b91e Mon Sep 17 00:00:00 2001
From: Tamal Das <72851613+IAmTamal@users.noreply.github.com>
Date: Wed, 4 Aug 2021 20:44:59 +0530
Subject: [PATCH] chore(CPlusPlus): add quick-select (#409)

---
 algorithms/CPlusPlus/Arrays/quick-select.cpp | 83 ++++++++++++++++++++
 algorithms/CPlusPlus/README.md               |  1 +
 2 files changed, 84 insertions(+)
 create mode 100644 algorithms/CPlusPlus/Arrays/quick-select.cpp

diff --git a/algorithms/CPlusPlus/Arrays/quick-select.cpp b/algorithms/CPlusPlus/Arrays/quick-select.cpp
new file mode 100644
index 00000000..c68c79a1
--- /dev/null
+++ b/algorithms/CPlusPlus/Arrays/quick-select.cpp
@@ -0,0 +1,83 @@
+// CPP program for implementation of "Quick-select" method based on Quick-sort or partition algorithm
+//The Question here is to find the kth smallest element in an array.
+//if array = {10 20 30 40 50 60} and k=2 then k'th smallest element will be : 20
+
+#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 left, int right)
+{
+    int x = arr[right], i = left;
+    for (int j = left; j <= right - 1; j++)
+    {
+        if (arr[j] <= x)
+        {
+            swap(arr[i], arr[j]);
+            i++;
+        }
+    }
+
+    swap(arr[i], arr[right]);
+    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 left, int right, int k)
+{
+    // If k is smaller than number of
+    // elements in array
+    if (k > 0 && k <= right - left + 1)
+    {
+
+        // Partition the array around last
+        // element and get position of pivot
+        // element in sorted array
+        int index = partition(arr, left, right);
+
+        // If position is same as k
+        if (index - left == k - 1)
+            return arr[index];
+
+        // If position is more, recur
+        // for left subarray
+        if (index - left > k - 1)
+            return kthSmallest(arr, left, index - 1, k);
+
+        // Else recur for right subarray
+        return kthSmallest(arr, index + 1, right,
+                           k - index + left - 1);
+    }
+
+    // If k is more than number of
+    // elements in array
+    return INT_MAX;
+}
+
+// Driver program to test above methods
+int main()
+{
+    int size;
+    cout << "Enter the size" << endl;
+    cin >> size;
+
+    int arr[size];
+    cout << "Enter the array" << endl;
+    for (int i = 0; i < size; i++)
+    {
+        cin >> arr[i];
+    }
+
+    int k;
+    cout << "Enter the value of k (must be less than size)" << endl;
+    cin >> k;
+    cout << "K-th smallest element is "
+         << kthSmallest(arr, 0, size - 1, k);
+    return 0;
+}
diff --git a/algorithms/CPlusPlus/README.md b/algorithms/CPlusPlus/README.md
index 9bb92107..d2d18786 100644
--- a/algorithms/CPlusPlus/README.md
+++ b/algorithms/CPlusPlus/README.md
@@ -11,6 +11,7 @@
 7. [Reverse Array](Arrays/reverse-array.cpp)
 8. [Sorted-Rotated Search Array](Arrays/search-sorted-rotated.cpp)
 9. [Fractional Knapsack](Arrays/fractional-knapsack.cpp)
+10. [Quick Selection](Arrays/quick-select.cpp)
 
 ## Dynamic-Programming