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select.java

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+// Deterministic select
+import java.util.Arrays;
+
+public class select {
+
+    public static int medianOfMedians(int[] arr) {
+        int n = arr.length;
+        int groupsCount = (n + 4) / 5;
+        int[] medians = new int[groupsCount];
+
+        for (int i = 0; i < groupsCount; i++) {
+            int start = i * 5;
+            int end = Math.min(start + 5, n);
+            int[] group = Arrays.copyOfRange(arr, start, end);
+            Arrays.sort(group);
+            medians[i] = group[group.length / 2];
+        }
+
+        if (medians.length <= 5) {
+            Arrays.sort(medians);
+            return medians[medians.length / 2];
+        } else {
+            return medianOfMedians(medians);
+        }
+    }
+
+    public static int partition(int[] arr, int low, int high, int pivot) {
+        int i = low;
+        for (int j = low; j < high; j++) {
+            if (arr[j] < pivot) {
+                int temp = arr[i];
+                arr[i] = arr[j];
+                arr[j] = temp;
+                i++;
+            }
+        }
+        int temp = arr[i];
+        arr[i] = arr[high];
+        arr[high] = temp;
+        return i;
+    }
+
+    public static int deterministicSelect(int[] arr, int low, int high, int k) {
+        if (low == high) {
+            return arr[low];
+        }
+
+        int pivot = medianOfMedians(Arrays.copyOfRange(arr, low, high + 1));
+
+        int pivotIndex = partition(arr, low, high, pivot);
+
+        if (pivotIndex == k) {
+            return arr[pivotIndex];
+        } else if (pivotIndex > k) {
+            return deterministicSelect(arr, low, pivotIndex - 1, k);
+        } else {
+            return deterministicSelect(arr, pivotIndex + 1, high, k);
+        }
+    }
+
+    public static void main(String[] args) {
+        int[] arr = {38, 27, 43, 3, 9, 82, 10};
+        int k = 3;
+        int result = deterministicSelect(arr, 0, arr.length - 1, k);
+        System.out.println("The " + (k + 1) + "-th smallest element is: " + result);
+    }
+}