PROCEDURE QSORT3(N:INTEGER) ;
    VAR L,I,X,W,R,S,J:INTEGER;
    BEGIN
      S:=1;
      STACK1[S]:=1 ;
      STACK2[S]:=N ;
      REPEAT
        L:=STACK1[S] ;
        R:=STACK2[S] ;
        S:=S-1;
        REPEAT
          I:=L;
          J:=R;
          X:=A[(L+R) DIV 2];
          REPEAT
            WHILE A[I]<X DO I:=I+1;
            WHILE A[J]>X DO J:=J-1;
            IF I<=J THEN BEGIN
              W:=A[I];
              A[I]:=A[J];
              A[J]:=W;
              I:=I+1;
              J:=J-1
            END;
          UNTIL I>J;
          IF I<R THEN BEGIN
            S:=S+1;
            STACK1[S]:=I ;
            STACK2[S]:=R ;
          END;
          R:=J;
        UNTIL L>=R;
      UNTIL S=0
    END ;