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

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

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+#include <iostream>
+#include <vector>
+#include <algorithm>
+
+using namespace std;
+
+// Function to partition the array and return the pivot index
+int partition(vector<int>& arr, int left, int right) {
+    int pivot = arr[right];
+    int i = left;
+
+    for (int j = left; j < right; ++j) {
+        if (arr[j] <= pivot) {
+            swap(arr[i], arr[j]);
+            i++;
+        }
+    }
+    swap(arr[i], arr[right]);
+    return i;
+}
+
+// Function to find the k-th smallest element using QuickSelect
+int quickSelect(vector<int>& arr, int left, int right, int k) {
+    if (left == right) return arr[left]; // Base case: only one element
+
+    int pivotIndex = partition(arr, left, right);
+
+    if (k == pivotIndex) {
+        return arr[k]; // Found the k-th smallest element
+    } else if (k < pivotIndex) {
+        return quickSelect(arr, left, pivotIndex - 1, k); // Search left
+    } else {
+        return quickSelect(arr, pivotIndex + 1, right, k); // Search right
+    }
+}
+
+int main() {
+    vector<int> arr = {3, 2, 1, 5, 6, 4};
+    int k = 2; // We want the 2nd smallest element
+
+    // Input validation for k
+    if (k < 1 || k > arr.size()) {
+        cout << "Error: k must be between 1 and " << arr.size() << endl;
+        return -1;
+    }
+
+    cout << k << "-th smallest element is " << quickSelect(arr, 0, arr.size() - 1, k - 1) << endl;
+    return 0;
+}