Top k問題

  /**
   * 最小的低k個數
   * 1. 建一個大根堆
   * 2. 若是比1個元素大，和第一個元素交換，調整堆
   * 3. 返回堆頂元素，即第一個元素
   * */
  private static int topMinK(int[] nums, int k) {
    buildMaxHeap(nums, k);
    for (int i = k + 1; i <= nums.length; i++) {
      if(nums[i - 1] < nums[0]) {
        nums[0] = nums[i - 1];
        downAdjust(nums, 1, k);
      }
    }
    return nums[0];
  }

  public static void heapSort(int[] nums) {
    buildMaxHeap(nums, nums.length);
    System.out.println("after build heap:");
    ArrayUtil.printIntArray(nums);
    for (int i = nums.length; i > 1; i--) {
      int temp = nums[i - 1];
      nums[i - 1] = nums[0];
      nums[0] = temp;
      downAdjust(nums, 1, i - 1);
    }
  }

  //10 8 5 12 9
  private static void buildMaxHeap(int[] nums, int end) {
    for (int i = end; i >= 1; i--) {
      downAdjust(nums, i, end);
    }
  }
  private static void downAdjust(int[] nums, int j, int end) {
      while (j * 2 <= end) {
        int maxIndex = j;
        maxIndex = j * 2 <= end && nums[j - 1] < nums[j * 2 - 1] ? j * 2 : maxIndex;
        maxIndex = j * 2 + 1 <= end && nums[maxIndex - 1] < nums[j * 2] ? j * 2 + 1 : maxIndex;
        if (maxIndex != j) {
          int temp = nums[maxIndex - 1];
          nums[maxIndex - 1] = nums[j - 1];
          nums[j - 1] = temp;
          j = maxIndex;
        } //if
        j = j * 2;
      } //while
  }