【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[0..210000]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:=0 to n-1 do
 begin
  x[i]:=a[i];
  inc(wc[a[i]]);
 end;
 for i:=1 to m-1 do wc[i]:=wc[i-1]+wc[i];
 for i:=n-1 downto 0 do
 begin
  dec(wc[x[i]]);
  sa[wc[x[i]]]:=i;
 end;
 j:=1; p:=1;
 while p<n do
 begin
  p:=0;
  for i:=n-j to n-1 do
  begin
   y[p]:=i; inc(p);
  end;
  for i:=0 to n-1 do
   if sa[i]>=j then begin y[p]:=sa[i]-j; inc(p); end;
  for i:=0 to n-1 do wd[i]:=x[y[i]];
  for i:=0 to m-1 do wc[i]:=0;
  for i:=0 to n-1 do inc(wc[wd[i]]);
  for i:=1 to m-1 do wc[i]:=wc[i-1]+wc[i];
  for i:=n-1 downto 0 do
  begin
   dec(wc[wd[i]]);
   sa[wc[wd[i]]]:=y[i];
  end;
  for i:=0 to n do swap(x[i],y[i]);
  p:=1; x[sa[0]]:=0;
  for i:=1 to n-1 do
   if cmp(sa[i-1],sa[i],j) then x[sa[i]]:=p-1
    else begin x[sa[i]]:=p; inc(p); end;
  j:=j*2;
  m:=p;
 end;
end;

procedure getheight(n:longint);
var i,j,k:longint;
begin
 for i:=1 to n do rank[sa[i]]:=i;
 k:=0;
 for i:=0 to n-1 do
 begin
  if k>0 then dec(k);
  j:=sa[rank[i]-1];
  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:=0 to l1-1 do a[i]:=ord(ch[i+1]);
 a[l1]:=1;
 readln(ch);
 l2:=length(ch);
 for i:=l1+1 to l1+l2 do a[i]:=ord(ch[i-l1]);
 n:=l1+l2+1; a[n]:=0; m:=300;
 getsa(n+1);
 getheight(n);
 inc(n);
 for i:=2 to n do
 begin
  l:=sa[i-1]+1; r:=sa[i]+1;
  if l>r then swap(l,r);
  if (l<=l1)and(r>=l1+2) then ans:=max(ans,height[i]);
 end;

 writeln(ans);

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