[BZOJ3524] [Poi2014]Couriers/[BZOJ2223] [Coci 2009]PATULJCI

传送门

http://www.lydsy.com/JudgeOnline/problem.php?id=3524
http://www.lydsy.com/JudgeOnline/problem.php?id=2223

题目大意

给定序列,询问区间内出现次数大于(r-l+1)/2的数

题解

主席树
我发现我主席树的MDbug

const
    maxn=500005;
var
    root:array[0..maxn]of longint;
    w:array[0..20*maxn,1..3]of longint; 
    i,j,k:longint;
    n,m,len,a,b,ans:longint;
procedure build(pre:longint;var now:longint; val,l,r:longint);
var mid:longint;
begin
    w[now,3]:=w[pre,3]+1; mid:=(l+r)>>1;
    if l=r then exit;
    if val<=mid
    then begin w[now,2]:=w[pre,2]; inc(len); w[now,1]:=len; build(w[pre,1],w[now,1],val,l,mid); end
    else begin w[now,1]:=w[pre,1]; inc(len); w[now,2]:=len; build(w[pre,2],w[now,2],val,mid+1,r) end;
end;

function query(pre,now,val,l,r:longint):longint;
var mid,t1,t2:longint;
begin
    if l=r then exit(l);
    mid:=(l+r)>>1;
    t1:=w[w[now,1],3]-w[w[pre,1],3];
    t2:=w[w[now,2],3]-w[w[pre,2],3];
    if t1>val then exit(query(w[pre,1],w[now,1],val,l,mid));
    if t2>val then exit(query(w[pre,2],w[now,2],val,mid+1,r));
    exit(0);
end;

begin
    fillchar(root,sizeof(root),0);
    fillchar(w,sizeof(w),0);
    readln(n,m); root[0]:=0; len:=0;
    for i:=1 to n do
        begin
            read(a); inc(len); root[i]:=len;
            build(root[i-1],root[i],a,1,n);
        end;
    for i:=1 to m do
        begin
            readln(a,b);
            ans:=query(root[a-1],root[b],(b-a+1)div 2,1,n);
            writeln(ans);
        end;
end.