POJ2454

题意:给出3*k个数,要求将其分为3个长度为k的序列,使得至少有两个序列的和大于500*k.

分析:贪心+随机化.首先排序将最小的前k个去掉,剩下的才最有可能满足题意.然后随机交换

[k+1,2k]和[2*k+1,3k]这两个区间内的数,知道满足题意,然后输出.

因为k只有60,直接暴力即可.

code:

type  rectype=record
      n,p:longint;
end;
var   c:array[0..181] of rectype;
      n,k,i,x,y:longint;

      procedure sort(l,r:longint);
      var   i,j,mid:longint;
      begin
            i:=l; j:=r;
            mid:=c[(l+r)>>1].n;
            while i<=j do
            begin
                  while c[i].n<mid do inc(i);
                  while c[j].n>mid do dec(j);
                  if i<=j then
                  begin
                        c[0]:=c[i];
                        c[i]:=c[j];
                        c[j]:=c[0];
                        inc(i); dec(j);
                  end;
            end;
            if i<r then sort(i,r);
            if j>l then sort(l,j);
      end;

      procedure swap(a,b:longint);
      var   tmp:rectype;
      begin
            tmp:=c[a]; c[a]:=c[b]; c[b]:=tmp;

      end;

      function OK:boolean;
      var   sum1,sum2:longint;
      begin
            sum1:=0;
            sum2:=0;
            for i:=k+1 to 2*k do inc(sum1,c[i].n);
            for i:=k*2+1 to n do inc(sum2,c[i].n);
            if (sum1>k*500)and(sum2>k*500) then exit(true);
            exit(false);
      end;

begin
      readln(k);
      n:=k*3;
      for i:=1 to n do
      begin
            readln(c[i].n);
            c[i].p:=i;
      end;
      sort(1,n);
      repeat
             x:=random(k)+1+k;
             y:=random(k)+1+k*2;
             swap(x,y);
      until OK;
      for i:=n downto 1 do writeln(c[i].p);
end.