NOIP2010 关押罪犯 （并查集）

若x,y有关系 将x与y的补集， y与x的补集建立关系

const maxn=;
      maxm=;
var eg:array[..maxm,..] of longint;
    f:array[..maxn*] of longint;
    i,j,m,n,x,y,z:longint;
procedure swap(var a,b:longint);
var c:longint;
begin
    c:=a;a:=b;b:=c;
end;
function find(x:longint):longint;
begin
    if f[x]=x then exit(x);
    f[x]:=find(f[x]);
    exit(f[x]);
end;
procedure sort(l,r:longint);
var i,j,x:longint;
begin
    i:=l; j:=r;
    x:=eg[(l+r) div ,];
    while i<=j do
    begin
        while eg[i,]>x do inc(i);
        while x>eg[j,] do dec(j);
        if i<=j then
        begin
            swap(eg[i,],eg[j,]);
            swap(eg[i,],eg[j,]);
            swap(eg[i,],eg[j,]);
            inc(i);
            dec(j);
        end;
    end;
    if i<r then sort(i,r);
    if l<j then sort(l,j);
end;
begin
    readln(n,m);
    for i:= to m do read(eg[i,],eg[i,],eg[i,]);
    for i:= to n* do f[i]:=i;
    sort(,m);
    for i:= to m do
    begin
        x:=find(eg[i,]);
        y:=find(eg[i,]);
        if x=y then
        begin
            writeln(eg[i,]);
            exit;
        end;
        f[x]:=find(eg[i,]+n);
        f[y]:=find(eg[i,]+n);
    end;
    writeln();
end.