• P3723 【[AH2017/HNOI2017]礼物】


    被某大佬指出这是多项式板子!?

    我们假设我们原始数列是(a_i, c_i), 旋转后的数列是(a_i, b_i),我们的增加量为x

    [sum_{i = 1}^n(a_i - b_i + x)^2 ]

    拆开平方得:

    [sum_{i = 1}^na_i^2+b_i^2+x^2+2*x*a_i-2*x*b_i-2*a_i*b_1 ]

    把这些东西分下类:

    [x^2*n+(sum_{i=1}^na_i^2+b_i^2)+2*x*(sum_{i = 1}^n a_i+b_i)+2*(sum_{i = 1}^na_i*b_i) ]

    发现(x)只有([-100, 100]),我们考虑枚举x,然后就只有最后一堆是未知的

    我们考虑怎么求最后一堆:两个值乘在一起的和,是不是和多项式有关系呢?

    但这并不是一个卷积的形式,但我们考虑把b反向,原式就变成了:(sum_{i=1}^na_i*b_{n-i+1})

    于是我们就可与愉快的用多项式来做这道题了。不会多项式?戳戳看?

    把a数组倍长,把b数组反向,于是这道题的式子就成了:(sum_{i=1}^na_{i+k}c_{n-i+1}),然后多项式的第(n+1-2*n)就分别代表(k)(0-n-1)的值了

    (Code:)

    #include<bits/stdc++.h>
    using namespace std;
    #define il inline
    #define re register
    #define int long long
    const double pi = acos(-1);
    #define inf 12345678900000000
    il int read() {
        re int x = 0, f = 1; re char c = getchar();
        while(c < '0' || c > '9') { if(c == '-') f = -1; c = getchar();}
        while(c >= '0' && c <= '9') x = x * 10 + c - 48, c = getchar();
        return x * f;
    }
    #define rep(i, s, t) for(re int i = s; i <= t; ++ i)
    #define maxn 300005
    struct node {
    	double x, y;
    }a[maxn], b[maxn];
    il node operator + (node a, node b) { return (node){a.x + b.x, a.y + b.y}; }
    il node operator - (node a, node b) { return (node){a.x - b.x, a.y - b.y}; }
    il node operator * (node a, node b) { return (node){a.x * b.x - a.y * b.y, a.x * b.y + a.y * b.x}; }
    int n, m, lim, r[maxn], ans = inf, sum1, sum2, Ans = -inf;
    il void FFT(node *a, int f, int len) {
    	rep(i, 0, len - 1) if(r[i] > i) swap(a[r[i]], a[i]);
    	for(re int mid = 1; mid < len; mid <<= 1) {
    		node base = (node){cos(pi / mid), f * sin(pi / mid)};
    		for(re int p = mid * 2, j = 0; j < len; j += p) {
    			node w = (node){1, 0};
    			for(re int k = 0; k < mid; ++ k, w = w * base) {
    				node x = a[j + k], y = a[j + k + mid] * w;
    				a[j + k] = x + y, a[j + k + mid] = x - y;
    			}
    		}
    	}
    }
    signed main() {
    	n = read(), m = read();
    	rep(i, 1, n) a[i].x = a[i + n].x = read();
    	rep(i, 1, n) b[n - i + 1].x = read();
    	rep(i, 1, n) sum1 += a[i].x * a[i].x + b[i].x * b[i].x, sum2 += a[i].x - b[i].x;
    	while((1 << lim) <= 3 * n) ++ lim;
    	rep(i, 0, (1 << lim)) r[i] = (r[i >> 1] >> 1) | ((i & 1) << (lim - 1));
    	FFT(a, 1, (1 << lim)), FFT(b, 1, (1 << lim));
    	rep(i, 0, (1 << lim)) a[i] = a[i] * b[i];
    	FFT(a, -1, (1 << lim));
    	rep(x, -m, m) ans = min(ans, x * x * n + sum1 + 2 * x * sum2);
    	rep(i, n + 1, 2 * n) Ans = max(Ans, (int)(a[i].x / (1 << lim) + 0.5));
    	printf("%lld", ans - 2 * Ans);
    	return 0;
    }
    
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  • 原文地址:https://www.cnblogs.com/bcoier/p/11774568.html
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