Heap Sort

/*
 * Problem : Sorted array.
 * Input   : n, then n integers.
 * Output  : The sorted array.
 * Approach: Max-heap in place.
 * Time    : O(n log n)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;
import java.util.Arrays;
import java.util.stream.Collectors;

public class Main {
        // Step 1: siftDown moves a value down until both children are not bigger. "size" is the length of the heap part.
        static void siftDown(int[] a, int start, int size) {
            int index = start;
            while (true) {
                int left = 2 * index + 1;
                int right = left + 1;
                int biggest = index;
                if (left < size && a[left] > a[biggest]) {
                    biggest = left;
                }
                if (right < size && a[right] > a[biggest]) {
                    biggest = right;
                }
                if (biggest == index) {
                    return;
                }
                int temporary = a[index];
                a[index] = a[biggest];
                a[biggest] = temporary;
                index = biggest;
            }
        }

    // This method solves the task. It receives the input and returns the answer.
    static int[] solve(int[] a) {
        int[] data = a.clone();
        int n = data.length;
        // Step 2: Build the max-heap. Leaves are already heaps, so start at the last parent and go back to the root.
        for (int i = n / 2 - 1; i >= 0; i--) {
            siftDown(data, i, n);
        }
        // Step 3: Move the biggest value to the end, make the heap one element smaller and repair it.
        for (int end = n - 1; end > 0; end--) {
            int temporary = data[0];
            data[0] = data[end];
            data[end] = temporary;
            siftDown(data, 0, end);
        }
        return data;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int size = scanner.nextInt();
        int[] a = new int[size];
        for (int i = 0; i < size; i++) {
            a[i] = scanner.nextInt();
        }
        System.out.println(Arrays.stream(solve(a)).mapToObj(String::valueOf).collect(Collectors.joining(" ")));
    }
}