bzoj3156

斜率优化dp，比较裸

注意int64的运算

var f,a,q:array[0..2000010] of int64;
    i,n,h,t:longint;
    x,y,z:int64;

function g(j,k:int64):double;
  var p,q:double;
  begin
    p:=2*(f[k]-f[j])+sqr(k)-sqr(j)-k+j;
    q:=2*(k-j);
    exit(p/q);
  end;

begin
  readln(n);
  for i:=1 to n do
    read(a[i]);
  h:=0;
  t:=0;
  q[0]:=n;
  f[n]:=a[n];
  for i:=n-1 downto 0 do
  begin
    while (h<t) and (g(q[h],q[h+1])>i) do inc(h);
    x:=q[h];
    y:=i;
    z:=(x-y)*(x-y-1);
    f[i]:=(2*f[q[h]]+z) div 2+a[i];
    if i<>0 then
    begin
      while (h<t) and (g(q[t],i)>g(q[t-1],q[t])) do dec(t);
      inc(t);
      q[t]:=i;
    end;
  end;
  writeln(f[0]);
end.