【BZOJ1717】[Usaco2006 Dec]Milk Patterns 产奶的模式 (二分+SA)

求重复k次的最长重复子串，解法见罗穗骞大神的后缀数组论文

 const maxn=;
 
 var
  x,y,rank,sa,h,s,num,c:array[..maxn] of longint;
  n,time:longint;
 
 function max(x,y:longint):longint; begin if x>y then exit(x) else exit(y); end;
 function min(x,y:longint):longint; begin if x<y then exit(x) else exit(y); end;
 procedure make;
 var i,j,p,tot:longint;
 begin
  p:=;
  while p<n do
   begin
    fillchar(c,sizeof(c),);
    for i:=  to n-p do y[i]:=rank[i+p];
    for i:= n-p+ to n do y[i]:=;
    for i:=  to n do inc(c[y[i]]);
    for i:=  to n do inc(c[i],c[i-]);
    for i:=  to n do
     begin
      sa[c[y[i]]]:=i;
      dec(c[y[i]]);
     end;
    fillchar(c,sizeof(c),);
    for i:=  to n do x[i]:=rank[i];
    for i:=  to n do inc(c[x[i]]);
    for i:=  to n do inc(c[i],c[i-]);
    for i:= n downto  do
     begin
      y[sa[i]]:=c[x[sa[i]]];
      dec(c[x[sa[i]]]);
     end;
    for i:=  to n do sa[y[i]]:=i;
    tot:=;
    rank[sa[]]:=;
    for i:=  to n do
     begin
      if (x[sa[i]]<>x[sa[i-]]) or (x[sa[i]+p]<>x[sa[i-]+p]) then inc(tot);
      rank[sa[i]]:=tot;
     end;
    p:=p<<;
   end;
 end;
 
 procedure makeht;
 var i,j,p:longint;
 begin
  h[]:=; p:=;
  for i:=  to n do
   begin
    p:=max(p-,);
    if rank[i]= then continue;
    j:=sa[rank[i]-];
    while (i+p<=n) and (j+p<=n) and (s[i+p]=s[j+p]) do inc(p);
    h[rank[i]]:=p;
   end;
 end;
 
 procedure init;
 var i,j,tot:longint;
  ch:char;
 begin
  readln(n,time);
  for i:=  to n do readln(s[i]);
  for i:=  to n do x[i]:=s[i];
  fillchar(c,sizeof(c),);
  for i:=  to n do inc(c[x[i]]);
  for i:=  to  do inc(c[i],c[i-]);
  for i:=  to n do
   begin
    sa[c[x[i]]]:=i;
    dec(c[x[i]]);
   end;
  rank[sa[]]:=;
  tot:=;
  for i:=  to n do
   begin
    if x[sa[i]]<>x[sa[i-]] then inc(tot);
    rank[sa[i]]:=tot;
   end;
  make;
  makeht;
 end;
 
 function check(k:longint):boolean;
 var i,j,x,y:longint;
 begin
  i:=;
  while i<=n do
   begin
    j:=i;
    while (j<n) and (h[j+]>=k) do inc(j);
    if j-i+>=time then exit(true);
    i:=j+;
   end;
  exit(false);
 end;
 
 procedure solve;
 var l,r,mid,ans:longint;
 begin
  l:=; r:=n>>; ans:=;
  while l<=r do
   begin
    mid:=(l+r)>>;
    if check(mid) then
     begin
      l:=mid+;
      ans:=mid;
     end
    else r:=mid-;
   end;
  writeln(ans);
 end;
 
 Begin
  init;
  solve;
 End.