使用Qsort(QuickSort)实现快速排序

最新推荐文章于 2023-08-07 17:25:33 发布

转载最新推荐文章于 2023-08-07 17:25:33 发布 · 708 阅读

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#pivot #integer #sorting #string #random #div

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本文提供了三种不同的快速排序算法实现方式，包括针对字符串列表、整数数组及自定义记录类型的排序过程。每种实现都详细展示了如何通过递归进行分区和交换元素以达到排序目的。

转载地址：http://www.codecodex.com/wiki/index.php?title=Quicksort

示例一、

procedure QuickSort(const AList: TStrings; const AStart, AEnd: Integer);
  procedure Swap(const AIdx1, AIdx2: Integer);
  var
    Tmp: string;
  begin
    Tmp := AList[AIdx1];
    AList[AIdx1] := AList[AIdx2];
    AList[AIdx2] := Tmp;
  end;

var
  Left: Integer;
  Pivot: string;
  Right: Integer;
begin
  if AStart >= AEnd then
    Exit;
  Pivot := AList[AStart];
  Left := AStart + 1;
  Right := AEnd;
  while Left < Right do
    begin
      if AList[Left] < Pivot then
        Inc(Left)
      else
        begin
          Swap(Left, Right);
          Dec(Right);
        end;
    end;
  Dec(Left);
  Swap(Left, AStart);
  Dec(Left);
  QuickSort(AList, AStart, Left);
  QuickSort(AList, Right, AEnd);
end;

示例二、

procedure QSort(var A: array of Integer);
  procedure QuickSort(var A: array of Integer; iLo, iHi: Integer);
  var Lo, Hi, Mid, T: Integer;
  begin
    Lo := iLo;
    Hi := iHi;
    Mid := A[(Lo + Hi) div 2];
    repeat
      while A[Lo] < Mid do
        Inc(Lo);
      while A[Hi] > Mid do
        Dec(Hi);
      if Lo <= Hi then begin
        T := A[Lo];
        A[Lo] := A[Hi];
        A[Hi] := T;
        Inc(Lo);
        Dec(Hi);
      end;
    until Lo > Hi;
    if Hi > iLo then
      QuickSort(A, iLo, Hi);
    if Lo < iHi then
      QuickSort(A, Lo, iHi);
  end;
begin
  QuickSort(A, Low(A), High(A));
end;

示例三、

type
  TCustomRecord = record
    Key: WideString;
    foo1:Int64;
    foo2:TDatetime;
    foo3:Extended;
  end;
  TCustomArray = array of TCustomRecord;

procedure QuickSort(var A: TCustomArray; L, R: Integer; var tmp: TCustomRecord);
var
  OrigL,
  OrigR: Integer;
  Pivot: WideString;
  GoodPivot,
  SortPartitions: Boolean;
begin
  if L<R then begin
    Pivot:=A[L+Random(R-L)].Key;
    OrigL:=L; //saving original bounds
    OrigR:=R;
    repeat
      L:=OrigL; //restoring original bounds if we
      R:=OrigR; //have chosen a bad pivot value
      while L<R do begin
        while (L<R) and (A[L].Key<Pivot) do Inc(L);
        while (L<R) and (A[R].Key>=Pivot) do Dec(R);
        if (L<R) then begin
          tmp:=A[L];
          A[L]:=A[R];
          A[R]:=tmp;
          Dec(R);
          Inc(L);
        end;
      end;
      if A[L].Key>=Pivot then Dec(L);                            //has we managed to choose
      GoodPivot:=L>=OrigL;                                       //a good pivot value?
      SortPartitions:=True;                                      //if so, then sort on
      if not GoodPivot then begin                                //bad luck, the pivot is the smallest one in our range
        GoodPivot:=True;                                         //let's presume that all the values are equal to pivot
        SortPartitions:=False;                                   //then no need to sort it
        for R := OrigL to OrigR do if A[R].Key<>Pivot then begin //we have at least one different value than our pivot
          Pivot:=A[R].Key;                                       //so this will be our new pivot
          GoodPivot:=False;                                      //we have to start again sorting this range
          Break;
        end;
      end;
    until GoodPivot;
    if SortPartitions then begin
      QuickSort(A, OrigL, L, tmp);
      QuickSort(A, L+1, OrigR, tmp);
    end;
  end;
end;