在CSDN的C/C++版看到这个算法问题：
测试代码：
- C# code
- public const int arrayLength = 30000; // 数组的长度private void button1_Click(object sender, EventArgs e)
{
#region 初始化数组
int[] array = new int[arrayLength];
Random random = new Random(1000); // 固定随机种子，使大家测试数据一致
for (int i = 0; i < array.Length; i++)
array[i] = random.Next(arrayLength 10);
#endregion 初始化数组
int A, B, C;
long tickCount = Environment.TickCount;
Search(array, out A, out B, out C);
Console.WriteLine("计算结果：A={0} B={1} C={2}，耗时:{3}",
A, B, C, Environment.TickCount - tickCount);
}
public void Search(int[] array, out int A, out int B, out int C)
{
A = -1;
B = -1;
C = -1;
/ TODO : 自由发挥 /
}
回帖一：
- C# code
- public void Search(int[] array, out int A, out int B, out int C)
{
A = -1;
B = -1;
C = -1;
int[] all = new int[arrayLength 10];
for (int i = 0; i < array.Length; i++)
all[array[i]]++;
for (int j = all.Length-1; j > 0; j–)
{
if (all[j] == 0) continue;
int num = j;
int halfnum=num/2;
if (num % 2 == 0)
{
if (all[halfnum] >= 2)
{
A = B = halfnum;
C = j;
return;
}
}
for (int t = 1; t < halfnum; t++)
{
if (all[t] > 0 && all[num - t] > 0)
{
A = t;
B = num - t;
C = num ;
return;
}
}
}
回帖二：
- C# code
static public void Search(int[] array, out int A, out int B, out int C)
{
A = -1;
B = -1;
C = -1;
if( array == null || array.Length < 3) return;
// O(nlog n)
Array.Sort<int>(array);
int maxNumber = array[ array.Length-1 ];
bool hasZero = (array[0] == 0);
unsafe
{
byte masks = stackalloc byte[maxNumber+2];
//if not Microsoft’s CLR, DO initialize the allocated stack!
//for (int i = 0; i < maxNumber + 2; i++) masks[i] = 0;
for(int i=0; i<array.Length; i++)
{
masks[ array[i] ]++;
}
// O(n*n)
for(int i=array.Length -1; i>=0; i–)
{
int sum = array[i];
// special case
if( hasZero && masks[sum] > 1 )
{
A = 0; B = sum; C = sum; return;
}
int halfSum = sum / 2;
for(int j = i-1; j>0 && array[j] >= halfSum; j–)
{
if (masks[sum - array[j]] > 0)
{
A = sum - array[j]; B = array[j]; C = sum;
if (A != B || masks[A] > 1) return;
else { A = B = C = -1; }
}
}
}
}
// done
}
以上代码还没仔细检查．