【POJ2774&Ural1517】Long Long Message（后缀数组）

题意：求两个字符串的最长公共子串

n<=1000

思路：这是一道论文题

 var x,y,sa,rank,wc,wd,a,height:array[..]of longint;
     n,l1,l2,i,ans,m,l,r:longint;
     ch:ansistring;

 procedure swap(var x,y:longint);
 var t:longint;
 begin
  t:=x; x:=y; y:=t;
 end;

 function max(x,y:longint):longint;
 begin
  if x>y then exit(x);
  exit(y);
 end;

 function cmp(a,b,l:longint):boolean;
 begin
  exit((y[a]=y[b])and(y[a+l]=y[b+l]));
 end;

 procedure getsa(n:longint);
 var i,j,p:longint;
 begin
  for i:= to n- do
  begin
   x[i]:=a[i];
   inc(wc[a[i]]);
  end;
  for i:= to m- do wc[i]:=wc[i-]+wc[i];
  for i:=n- downto  do
  begin
   dec(wc[x[i]]);
   sa[wc[x[i]]]:=i;
  end;
  j:=; p:=;
  while p<n do
  begin
   p:=;
   for i:=n-j to n- do
   begin
    y[p]:=i; inc(p);
   end;
   for i:= to n- do
    if sa[i]>=j then begin y[p]:=sa[i]-j; inc(p); end;
   for i:= to n- do wd[i]:=x[y[i]];
   for i:= to m- do wc[i]:=;
   for i:= to n- do inc(wc[wd[i]]);
   for i:= to m- do wc[i]:=wc[i-]+wc[i];
   for i:=n- downto  do
   begin
    dec(wc[wd[i]]);
    sa[wc[wd[i]]]:=y[i];
   end;
   for i:= to n do swap(x[i],y[i]);
   p:=; x[sa[]]:=;
   for i:= to n- do
    if cmp(sa[i-],sa[i],j) then x[sa[i]]:=p-
     else begin x[sa[i]]:=p; inc(p); end;
   j:=j*;
   m:=p;
  end;
 end;

 procedure getheight(n:longint);
 var i,j,k:longint;
 begin
  for i:= to n do rank[sa[i]]:=i;
  k:=;
  for i:= to n- do
  begin
   if k> then dec(k);
   j:=sa[rank[i]-];
   while a[i+k]=a[j+k] do inc(k);
   height[rank[i]]:=k;
  end;
 end;

 begin
  assign(input,'poj2774.in'); reset(input);
  assign(output,'poj2774.out'); rewrite(output);
  readln(ch);
  l1:=length(ch);
  for i:= to l1- do a[i]:=ord(ch[i+]);
  a[l1]:=;
  readln(ch);
  l2:=length(ch);
  for i:=l1+ to l1+l2 do a[i]:=ord(ch[i-l1]);
  n:=l1+l2+; a[n]:=; m:=;
  getsa(n+);
  getheight(n);
  inc(n);
  for i:= to n do
  begin
   l:=sa[i-]+; r:=sa[i]+;
   if l>r then swap(l,r);
   if (l<=l1)and(r>=l1+) then ans:=max(ans,height[i]);
  end;

  writeln(ans);

  close(input);
  close(output);
 end.