# BZOJ 1010. [HNOI2008]玩具装箱toy

### Analysis:

$$! f_i = \min_{0 <= j < i}\big\{ f_j + (i-j+s_i-s_{j-1}-L)^2 | j < i\big\}$$ 观察，微调，分离常数。 [cpp] REP_1(i, n) s[i] += i; ++L; [/cpp] ! \begin{aligned} f_i &= \min_{0 \leq j < i}\big\{ f_j + ((s_i-L)-s_j)^2 \big\} \\ &= \min_{0 \leq j < i}\big\{ -2(s_i-L)s_j + (f_j+s_j^2) \big\} + (s_i-L)^2 \end{aligned} 写成斜率优化的标准形式，b = kx + y 这里有：

• $$k = -2(s_i-L)$$
• $$x = s_j$$
• $$y = f_j + x^2$$

//}/* .................................................................................................................................. */

const int N = int(5e4) + 9;

LL f[N], s[N]; int q[N], cz, op;
int n, L;

#define k (-2*(s[i]-L))
#define x(i) (s[i])
#define y(i) (f[i]+sqr(x(i)))
#define eval(i) (k*x(i)+y(i))

LL det(LL x1, LL y1, LL x2, LL y2){
return x1*y2 - x2*y1;
}

int dett(int p0, int p1, int p2){
LL t = det(x(p1)-x(p0), y(p1)-y(p0), x(p2)-x(p0), y(p2)-y(p0));
return t < 0 ? -1 : t > 0;
}

int main(){

#ifndef ONLINE_JUDGE
freopen("in.txt", "r", stdin);
freopen("out.txt", "w", stdout);
#endif

RD(n, L); REP_1(i, n) s[i] = s[i-1] + RD();
REP_1(i, n) s[i] += i; ++L;

cz = 0, op = 0; q[cz] = 0; REP_1(i, n){
while (cz < op && eval(q[cz]) >= eval(q[cz+1])) ++cz;
f[i] = eval(q[cz]) + sqr(s[i]-L);
while (cz < op && dett(q[op-1], q[op], i) <= 0) --op;
q[++op] = i;
}

OT(f[n]);
}