堆排序工具类（适用于top k问题，java泛型实现）

代码如下，作用如标题所述

 public class HeapSort {
     //方法作用:取出list里面的最小的 k 个值
     public static <T extends Comparable<T>> List<T> sort(List<T> list, int k) throws Exception {
         if (k <= 0) {
             throw new Exception("k 必须大于0");
         }
         if (list.size() < k) {
             throw new Exception("list 长度必须大于k");
         }
         List<T> heapList = new ArrayList<T>(k);
         for (int i = 0; i < k; i ++) {
             heapList.add(list.get(i));
         }
         initialHeap(heapList);
         for (int i = k; i < list.size(); i ++) {
             if (list.get(i).compareTo(heapList.get(0)) < 0) {
                 heapList.set(0, list.get(i));
                 heapify(heapList, k, 0);
             }
         }
         return heapList;
     }
     private static <T extends Comparable<T>> void initialHeap(List<T> list) {
         int n = list.size();
         // Build heap (rearrange array)
         for (int i = n / 2 - 1; i >= 0; i--)
             heapify(list, n, i);
     }
     private static <T extends Comparable<T>> void heapify(List<T> list, 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 && (list.get(l).compareTo(list.get(largest)) > 0))
             largest = l;
 
         // If right child is larger than largest so far
         if (r < n && (list.get(r).compareTo(list.get(largest)) > 0))
             largest = r;
 
         // If largest is not root
         if (largest != i)
         {
             T swap = list.get(i);
             list.set(i, list.get(largest));
             list.set(largest, swap);
             // Recursively heapify the affected sub-tree
             heapify(list, n, largest);
         }
     }
 }