Add Longest Common Subsequence in Pascal (TheRenegadeCoder#4836)

Author: pascalecu

archive/p/pascal/longest_common_subsequence.pas

@@ -0,0 +1,144 @@
+program LongestCommonSubsequence;
+
+{$mode objfpc}{$H+}
+
+uses
+   Generics.Collections,
+   SysUtils;
+
+type
+   TIntegerList = specialize TList<integer>;
+
+procedure ShowUsage;
+begin
+   Writeln('Usage: please provide two lists in the format "1, 2, 3, 4, 5"');
+   Halt(1);
+end;
+
+function ParseIntegerList(const S: string): TIntegerList;
+var
+   Tokens: TStringArray;
+   Token: string;
+   Value: integer;
+begin
+   if S.Trim = '' then
+      ShowUsage;
+
+   Tokens := S.Split([',']);
+   if Length(Tokens) = 0 then
+      ShowUsage;
+
+   Result := TIntegerList.Create;
+   for Token in Tokens do
+   begin
+      if not TryStrToInt(Trim(Token), Value) then
+      begin
+         Result.Free;
+         ShowUsage;
+      end;
+      Result.Add(Value);
+   end;
+end;
+
+
+function FormatIntegerList(const List: TIntegerList): string;
+var
+   i: integer;
+   Builder: TStringBuilder;
+begin
+   Builder := TStringBuilder.Create;
+   try
+      for i := 0 to List.Count - 1 do
+      begin
+         if i > 0 then
+            Builder.Append(', ');
+         Builder.Append(IntToStr(List[i]));
+      end;
+      Result := Builder.ToString;
+   finally
+      Builder.Free;
+   end;
+end;
+
+function LongestCommonSubsequence(const A, B: TIntegerList): TIntegerList;
+var
+   PreviousRow, CurrentRow: array of integer;
+   i, j: integer;
+begin
+   if B.Count > A.Count then
+      Exit(LongestCommonSubsequence(B, A));
+
+   SetLength(PreviousRow, B.Count + 1);
+   SetLength(CurrentRow, B.Count + 1);
+
+   // Build the DP table
+   for i := 1 to A.Count do
+   begin
+      for j := 1 to B.Count do
+         if A[i - 1] = B[j - 1] then
+            CurrentRow[j] := PreviousRow[j - 1] + 1
+         else
+         if CurrentRow[j - 1] > PreviousRow[j] then
+            CurrentRow[j] := CurrentRow[j - 1]
+         else
+            CurrentRow[j] := PreviousRow[j];
+
+      PreviousRow := CurrentRow;
+      SetLength(CurrentRow, B.Count + 1);
+   end;
+
+   // Reconstruct the LCS from the DP table
+   Result := TIntegerList.Create;
+   i := A.Count;
+   j := B.Count;
+
+   while (i > 0) and (j > 0) do
+      if A[i - 1] = B[j - 1] then
+      begin
+         // If they match, append
+         Result.Add(A[i - 1]);
+         Dec(i);
+         Dec(j);
+      end
+      else if PreviousRow[j] > CurrentRow[j - 1] then
+         // Move up if the top cell has a greater value
+         Dec(i)
+      else
+         // Move left if the left cell has a greater value
+         Dec(j);
+
+   Result.Reverse;
+end;
+
+
+var
+   ListA, ListB, LCSResult: TIntegerList;
+   i: integer;
+begin
+   if ParamCount <> 2 then
+      ShowUsage;
+
+   try
+      try
+         ListA := ParseIntegerList(ParamStr(1));
+         ListB := ParseIntegerList(ParamStr(2));
+
+         if (ListA.Count = 0) or (ListB.Count = 0) then
+            ShowUsage;
+
+         LCSResult := LongestCommonSubsequence(ListA, ListB);
+
+         if LCSResult.Count = 0 then
+            Writeln('No common subsequence.')
+         else
+            Writeln(FormatIntegerList(LCSResult));
+      except
+         on E: Exception do
+            ShowUsage;
+      end;
+   finally
+      ListA.Free;
+      ListB.Free;
+      LCSResult.Free;
+   end;
+end.