堆排序 Heap Sort

Heap Sort
使用heap sort做升序

1) 根据输入的数据创建最大堆；
2)创建以后最大的元素会在根节点上，交换根节点与最后一个元素，将剩余的元素重新堆化；
3) 重复2) 直到所有元素堆化完成。

Code

public class HeapSort {
    @Test
    public void test(){
        int[] arr = {4,10,3,5,1};
        sort(arr);
    }
    public void sort(int arr[]) {
        int n = arr.length;

        // Build heap (rearrange array)
        for (int i = n / 2 - 1; i >= 0; i--) {
            heapify(arr, n, i);
        }
        // One by one extract an element from heap
        for (int i = n - 1; i >= 0; i--) {
            // Move current root to end
            int temp = arr[0];
            arr[0] = arr[i];
            arr[i] = temp;
            // call max heapify on the reduced heap
            heapify(arr, i, 0);
        }
    }

    // To heapify a subtree rooted with node i which is
    // an index in arr[]. n is size of heap
    void heapify(int arr[], int n, int i) {
        int largest = i;  // Initialize largest as root
        int l = 2 * i + 1;  // left = 2*i + 1
        int r = 2 * i + 2;  // right = 2*i + 2

        // If left child is larger than root
        if (l < n && arr[l] > arr[largest]){
            largest = l;
        }
        // If right child is larger than largest so far
        if (r < n && arr[r] > arr[largest]){
            largest = r;
        }
        // If largest is not root
        if (largest != i) {
            int swap = arr[i];
            arr[i] = arr[largest];
            arr[largest] = swap;
            // Recursively heapify the affected sub-tree
            heapify(arr, n, largest);
        }
    }
}

Note

• 堆排序不需要额外空间；
• 在实际应用中，堆排序不如Quicksort 和Mergesort 那么广泛，但是二叉堆的数据结构有非常广泛的应用。