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x01.Weiqi.11: 神来之笔

在围棋中，一子两用，可谓妙手，而一子三用，则可称之为神来之笔。在解决征子问题时，一不小心，也来了个神来之笔，其代码如下：

// 征子判断，p1, p2 为气，p2 为前进方向，p 为逃跑之子。 bool CanLevy(Pos p1, Pos p2, Pos p, bool isBlack = true) {  if (!IsCusp(p1, p2)) return true;  if (p == m_InvalidPos) return true;  List<Pos> selfPoses = isBlack ? BlackPoses : WhitePoses;  List<Pos> otherPoses = !isBlack ? BlackPoses : WhitePoses;  // 征而被叫，岂不大笑？  var p1_links = LinkPoses(p1).Intersect(otherPoses).ToList();  if (p1_links.Count == 1 && p1_links.Intersect(EmptyPoses).Count() == 2)   return false;  var p2_links = LinkPoses(p2).Intersect(otherPoses).ToList();  if (p2_links.Count == 1 && p2_links.Intersect(EmptyPoses).Count() == 2)   return false;  int count = 0;  while (true) {   if (!InRange(p2.Row, p2.Col))    break;   bool isRow = p2.Row - p.Row == 0 ? true : false;   int rowOffset = isRow ? (count == 0 ? p1.Row - p2.Row : p2.Row - p1.Row) : 0;   int colOffset = isRow ? 0 : (count ==  0 ? p1.Col - p2.Col :p2.Col - p1.Col);   Pos pos = new Pos(p2.Row + rowOffset, p2.Col + colOffset);   var rounds = count < 5 ? LinkPoses(pos) : RoundTwoPoses(pos);   foreach (var r in rounds) {    if (isBlack && count < 2) continue; // 黑需先走两步    if (selfPoses.Contains(r))     return false;    if (otherPoses.Contains(r)) {     return true;    }   }   count++;   p1 = p;   p = p2;   p2 = pos;  }  return true; }