USACO 1.1.4 beads O(n)的两种算法

 1 type
 2   node=record
 3   bl,br,rl,rr,sum:longint;
 4   data:char;
 5 end;
 6 var
 7   len,i,j,t,mmax:longint;
 8   a:array[0..705]of node;
 9 function max(x,y:longint):longint;
10 begin
11   if x>y then exit(x)
12   else exit(y);
13 end;
14 begin
15   readln(len);
16   for i:=1 to len do
17     read(a[i].data);
18   for i:=1 to len do
19     a[i+len].data:=a[i].data;
20 
21   for i:=1 to len*2 do
22   case a[i].data of
23     ‘w‘:begin a[i].bl:=a[i-1].bl+1;a[i].rl:=a[i-1].rl+1;end;
24     ‘r‘:a[i].rl:=a[i-1].rl+1;
25     ‘b‘:a[i].bl:=a[i-1].bl+1;
26   end;
27 
28   for i:=len*2 downto 1 do
29   case a[i].data of
30     ‘w‘:begin a[i].br:=a[i+1].br+1;a[i].rr:=a[i+1].rr+1;end;
31     ‘r‘:a[i].rr:=a[i+1].rr+1;
32     ‘b‘:a[i].br:=a[i+1].br+1;
33   end;
34 
35   for i:=1 to len*2 do
36   begin
37     a[i].sum:=max(a[i-1].bl,a[i-1].rl)+max(a[i].br,a[i].rr);
38     if a[i].sum>len then a[i].sum:=len;
39     if a[i].sum>mmax then
40       mmax:=a[i].sum;
41   end;
42   writeln(mmax);
43 end.//具体什么的见nocow http://www.nocow.cn/index.php/USACO/beads思路一前面