/// <summary>
/// 在给定的一组数据中，所有和为0的整数对的数量
/// </summary>
/// <param name="a"></param>
/// <returns></returns>
public static int TwoSumCount(int[] a)
{
Array.Sort(a);
int n = a.Length;
int count = 0;
for (int i = 0; i < n; i++)
{
if (Foreword.Rank(-a[i], a) > i)
{
count++;
}
}
return(count);
}
/// <summary>
/// 所有三个数和为0的组合数
/// </summary>
/// <param name="a"></param>
/// <returns></returns>
public static int ThreeSumCount(int[] a)
{
Array.Sort(a);
int n = a.Length;
int count = 0;
for (int i = 0; i < n; i++)
{
for (int j = i + 1; j < n; j++)
{
if (Foreword.Rank(-(a[i] + a[j]), a) > j)
{
count++;
}
}
}
return(count);
}
/// <summary>
/// 返回大小为参数m的数组，其中第i个元素的值为整数i在参数数组中出现的次数
/// </summary>
/// <param name="array"></param>
/// <param name="m"></param>
/// <returns></returns>
public static int[] Histogram(int[] array, int m)
{
int[] result = new int[m];
Array.Sort(array);//先对该数组排序
for (int i = 0; i < m; i++)
{
int index = Foreword.Rank(i + 1, array);//用二分查找法找出该数在数组中的位置
if (index == -1)
{
result[i] = 0;
}
else
{
result[i] = GetCountInSortedArray(array, index);
}
}
return(result);
}