《Delphi 算法与数据结构》学习与感悟[1]: 通过 "顺序查找" 与 "二分查找" 说明算法的重要性

测试效果图:

unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm1 = class(TForm)
Button1: TButton;
Memo1: TMemo;
procedure Button1Click(Sender: TObject);
end;

var
Form1: TForm1;

implementation

{$R *.dfm}

{顺序查找函数}
function SeqSearch(List: TStringList; const str: string): Integer;
var
i: Integer;
begin
for i := 0 to List.Count - 1 do
if CompareText(List[i], str) = 0 then begin Result := i; Exit; end;
Result := -1;
end;

{二分查找函数; 二分查找只能针对有序列表}
function BinarySearch(List: TStringList; const str: string): Integer;
var
L,R,M: Integer;
CompareResult: Integer;
begin
Result := -1;
L := 0;
R := List.Count - 1;

while L <= R do
begin
M := (L + R) div 2;
CompareResult := CompareText(List[M], str);
if CompareResult < 0 then L := M + 1 else
if CompareResult > 0 then R := M - 1 else
begin
Result := M;
Exit;
end;
end;
end;

{对比测试}
procedure TForm1.Button1Click(Sender: TObject);
var
TestList: TStringList;
i: Integer;
n1,n2: Int64;
Count1,Count2: Integer;
s: string;
const
num = 1000000; {准备测试百万个数据}
begin
TestList := TStringList.Create;
for i := 0 to num-1 do TestList.Add(IntToHex(i,8)); {准备有序的测试值列表}

Memo1.Clear;
Count1 := 0;
Count2 := 0;

{搞 10 实验}
for i := 0 to 9 do
begin
{产生范围内的随机字串}
Randomize;
s := IntToHex(Random(num),8);

{顺序查找}
QueryPerformanceCounter(n1);
SeqSearch(TestList, s);
QueryPerformanceCounter(n2);
Memo1.Lines.Add(IntToStr(n2-n1)+ #9);
Count1 := Count1 + (n2-n1);

{二分查找}
QueryPerformanceCounter(n1);
BinarySearch(TestList, s);
QueryPerformanceCounter(n2);
Memo1.Lines[i] := Memo1.Lines[i] + IntToStr(n2-n1);
Count2 := Count2 + (n2-n1);
end;

Memo1.Lines.Add('----------------');
Memo1.Lines.Add('平均值:');
Memo1.Lines.Add(IntToStr(Count1 div 10)+ #9 + IntToStr(Count2 div 10));
Memo1.Lines.Add('----------------');
Memo1.Lines.Insert(0, '顺序'#9'二分');

TestList.Free;
end;

end.

二分查找太快了, 用 GetTickCount 测试不出来, 只好使用 QueryPerformanceCounter;

另外 TStringList.Find 方法也是使用了 "二分查找" 的办法.