给出类似＆＃34; N00MNM＆＃34;的字符串。我需要零＆＃39; 0＆＃39;的所有排列。字符串中的char以固定顺序维护所有其他字符。
结果必须是：
＆＃34; N0M0NM＆＃34; ＆＃34; N0MN0M＆＃34; ＆＃34; N0MNM0＆＃34; ＆＃34; NM00NM＆＃34; ＆＃34; NM0N0M＆＃34; ＆＃34; NM0NM0＆＃34; ＆＃34; NMN0M0＆＃34; ＆＃34; NMNM00＆＃34; ＆＃34; 0N0MNM＆＃34; ＆＃34; 0NM0NM＆＃34; ＆＃34; 0NMN0M＆＃34; ＆＃34; 0NMNM0＆＃34;
标准排列函数花费了太多时间来完成这项工作（我们谈论的是大约1500毫秒）并且要测试的字符串比样本字符串长。
有这个算法吗？
答案 0 :(得分：0)
您可以通过获取可以放置字符0（在这种情况下）的所有不同位置，然后包括0个字符的总数来完成您要做的事情（{{ 1}}在这种情况下）在字符串的所有位置。这些位置取自字符串而不会出现00。下面的代码是这样做的：
0 public static IEnumerable<string> Combs(string str, char c)
{
int count = str.Count(_c => _c == c);
string _str = new string(str.Where(_c => _c != c).ToArray());
// Compute all combinations with different positions
foreach (var positions in GetPositionsSets(0, _str.Length, count))
{
StringBuilder _b = new StringBuilder();
int index = 0;
foreach (var _char in _str)
{
if (positions.Contains(index))
{ _b.Append($"{c}{_char}"); }
else
{ _b.Append(_char); }
index++;
}
if (positions.Contains(index))
_b.Append(c);
yield return _b.ToString();
}
//Compute the remaining combinations. I.e., those whose at some position
//have the amount of supplied characters.
string p = new string(c, count);
for (int i = 0; i < _str.Length; i++)
{
yield return _str.Insert(i, p);
}
yield return _str + p;
}
//Gets all posible positions sets that can be obtain from minPos
//until maxPos with positionsCount positions, that is, C(n,k)
//where n = maxPos - minPos + 1 and k = positionsCount
private static IEnumerable<HashSet<int>> GetPositionsSets(int minPos, int maxPos, int positionsCount)
{
if (positionsCount == 0)
yield return new HashSet<int>();
for (int i = minPos; i <= maxPos; i++)
{
foreach (var positions in GetPositionsSets(i + 1, maxPos, positionsCount - 1))
{
positions.Add(i);
yield return positions;
}
}
}
上面代码的输出是：
"N00MNM"