Update Quickselect for Finding the k-th Smallest Element.cpp

Summary of the Code
** Partition Function: Splits the array and returns the pivot index.
** QuickSelect Function: Uses the partition function recursively to find the k-th smallest element.
** Test Function: Contains test cases using assert() to verify the correctness of the quickSelect function.
** Main Function: Runs tests and uses the quickSelect function to find the k-th smallest element from the input vector.

Author: dev-priyanshu15

search/Quickselect for Finding the k-th Smallest Element.cpp

@@ -1,10 +1,22 @@
 #include <iostream>
 #include <vector>
 #include <algorithm>
+#include <cassert>
 
 using namespace std;
 
-// Function to partition the array and return the pivot index
+/**
+ * @brief Partition the array and return the pivot index.
+ * 
+ * This function rearranges the elements in the array such that elements
+ * less than or equal to the pivot are on the left, and elements greater
+ * than the pivot are on the right.
+ * 
+ * @param arr The array of integers to partition.
+ * @param left The starting index of the segment to partition.
+ * @param right The ending index of the segment to partition.
+ * @returns The index of the pivot after partitioning.
+ */
 int partition(vector<int>& arr, int left, int right) {
     int pivot = arr[right];
     int i = left;
@@ -19,7 +31,18 @@ int partition(vector<int>& arr, int left, int right) {
     return i;
 }
 
-// Function to find the k-th smallest element using QuickSelect
+/**
+ * @brief Find the k-th smallest element using QuickSelect algorithm.
+ * 
+ * This function uses a recursive approach to partition the array and find
+ * the k-th smallest element.
+ * 
+ * @param arr The array of integers to search.
+ * @param left The starting index of the segment to search.
+ * @param right The ending index of the segment to search.
+ * @param k The index of the k-th smallest element (0-based).
+ * @returns The k-th smallest element in the array.
+ */
 int quickSelect(vector<int>& arr, int left, int right, int k) {
     if (left == right) return arr[left]; // Base case: only one element
 
@@ -34,7 +57,41 @@ int quickSelect(vector<int>& arr, int left, int right, int k) {
     }
 }
 
-int main() {
+/**
+ * @brief Self-test implementations
+ * 
+ * This function contains test cases to validate the QuickSelect algorithm
+ * using assertions.
+ * 
+ * @returns void
+ */
+static void test() {
+    vector<int> testArr1 = {3, 2, 1, 5, 6, 4};
+    assert(quickSelect(testArr1, 0, testArr1.size() - 1, 0) == 1);
+    assert(quickSelect(testArr1, 0, testArr1.size() - 1, 1) == 2);
+    assert(quickSelect(testArr1, 0, testArr1.size() - 1, 2) == 3);
+    assert(quickSelect(testArr1, 0, testArr1.size() - 1, 3) == 4);
+    assert(quickSelect(testArr1, 0, testArr1.size() - 1, 4) == 5);
+    assert(quickSelect(testArr1, 0, testArr1.size() - 1, 5) == 6);
+
+    vector<int> testArr2 = {10, 4, 5, 8, 6, 11, 26};
+    assert(quickSelect(testArr2, 0, testArr2.size() - 1, 0) == 4);
+    assert(quickSelect(testArr2, 0, testArr2.size() - 1, 3) == 8);
+    assert(quickSelect(testArr2, 0, testArr2.size() - 1, 6) == 26);
+
+    cout << "All tests have successfully passed!\n";
+}
+
+/**
+ * @brief Main function
+ * 
+ * @param argc Command-line argument count (ignored)
+ * @param argv Command-line array of arguments (ignored)
+ * @returns 0 on exit
+ */
+int main(int argc, char *argv[]) {
+    test();  // Run self-test implementations
+
     vector<int> arr = {3, 2, 1, 5, 6, 4};
     int k = 2; // We want the 2nd smallest element