CF1096G.Lucky Tickets[DP+卷积+多项式求幂] Educational Codeforces Round 57

清疚 2022-01-06 14:09 149阅读 0赞

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给定一个偶数长度n和字符集(0..9中的一些数字) 问有多少个串的前 \\(\\frac\{n\}\{2\}\\) 位的位数和跟后 \\(\\frac\{n\}\{2\}\\) 位相等

\\\[ f\\left( i,j \\right) \\text\{表示\}i\\text\{个数的和是\}j\\text\{的方案数\} \\\\ \\text\{答案是\}\\sum\_y\{f\\left( \\frac\{n\}\{2\},y \\right)\}^2\\text\{（前半部分和后半部分都是\}\\sum\_y\{f\\left( \\frac\{n\}\{2\},y \\right)\}\\text\{）\} \\\\ \\text\{所以只需要考虑怎么求\}f \\\\ f\\left( x+\\text\{1,\}y \\right) =\\sum\_\{i=0\}^9\{f\\left( x,y-i \\right)\} \\\\ \\text\{可以把上面的柿子补成卷积形式\} \\\\ \\text\{令\}H=f\\left( x+1 \\right) \\text\{，\}F=f\\left( x \\right) \\text\{，\}H\\left( y \\right) \\text\{表示的就是\}f\\left( x+\\text\{1,\}y \\right) \\text\{，\}F\\text\{同理\} \\\\ H\\left( k \\right) =\\sum\_\{i=0\}^\{\\min \\left( k,9 \\right)\}\{F\\left( i \\right)\}\\ast G\\left( k-i \\right) \\\\ \\text\{即\}f\\left( x+1 \\right) \\left( k \\right) =\\sum\_\{i=0\}^\{\\min \\left( k,9 \\right)\}\{f\\left( x \\right) \\left( i \\right)\}\\ast G\\left( k-i \\right) \\\\ \\text\{比较容易发现没有字符集限制的情况下\}G=1 \\\\ \\text\{有限制的时候\}G=\\sum\_\{i=0\}^9\{a\_ix^i\}\\text\{，\}a\_i\\text\{表示\}i\\text\{是否在允许的字符集中，\}x\\text\{没有实际意义\} \\\\ \\text\{所以要做的就是求出\}G\\left( x \\right) ^\{\\frac\{n\}\{2\}\}\\text\{然后统计每一项系数的平方和\} \\\]

贴个Tutorial里的代码

#include<bits/stdc++.h> using namespace std; const int LOGN = 21; const int N = (1 << LOGN); const int MOD = 998244353; const int g = 3; #define forn(i, n) for(int i = 0; i < int(n); i++) inline int mul(int a, int b) { return (a * 1ll * b) % MOD; } inline int norm(int a) { while(a >= MOD) a -= MOD; while(a < 0) a += MOD; return a; } inline int binPow(int a, int k) { int ans = 1; while(k > 0) { if(k & 1) ans = mul(ans, a); a = mul(a, a); k >>= 1; } return ans; } inline int inv(int a) { return binPow(a, MOD - 2); } vector<int> w[LOGN]; vector<int> iw[LOGN]; vector<int> rv[LOGN]; void precalc() { int wb = binPow(g, (MOD - 1) / (1 << LOGN)); for(int st = 0; st < LOGN; st++) { w[st].assign(1 << st, 1); iw[st].assign(1 << st, 1); int bw = binPow(wb, 1 << (LOGN - st - 1)); int ibw = inv(bw); int cw = 1; int icw = 1; for(int k = 0; k < (1 << st); k++) { w[st][k] = cw; iw[st][k] = icw; cw = mul(cw, bw); icw = mul(icw, ibw); } rv[st].assign(1 << st, 0); if(st == 0) { rv[st][0] = 0; continue; } int h = (1 << (st - 1)); for(int k = 0; k < (1 << st); k++) rv[st][k] = (rv[st - 1][k & (h - 1)] << 1) | (k >= h); } } inline void fft(int a[N], int n, int ln, bool inverse) { for(int i = 0; i < n; i++) { int ni = rv[ln][i]; if(i < ni) swap(a[i], a[ni]); } for(int st = 0; (1 << st) < n; st++) { int len = (1 << st); for(int k = 0; k < n; k += (len << 1)) { for(int pos = k; pos < k + len; pos++) { int l = a[pos]; int r = mul(a[pos + len], (inverse ? iw[st][pos - k] : w[st][pos - k])); a[pos] = norm(l + r); a[pos + len] = norm(l - r); } } } if(inverse) { int in = inv(n); for(int i = 0; i < n; i++) a[i] = mul(a[i], in); } } int aa[N], bb[N], cc[N]; inline void multiply(int a[N], int sza, int b[N], int szb, int c[N], int &szc) { int n = 1, ln = 0; while(n < (sza + szb)) n <<= 1, ln++; for(int i = 0; i < n; i++) aa[i] = (i < sza ? a[i] : 0); for(int i = 0; i < n; i++) bb[i] = (i < szb ? b[i] : 0); fft(aa, n, ln, false); fft(bb, n, ln, false); for(int i = 0; i < n; i++) cc[i] = mul(aa[i], bb[i]); fft(cc, n, ln, true); szc = n; for(int i = 0; i < n; i++) c[i] = cc[i]; } vector<int> T[N]; int a[N]; int b[N]; int c[N]; #define sz(a) (int(a.size())) int main() { precalc(); int n, k; scanf("%d %d", &n, &k); for(int i = 0; i < k; i++) { int x; scanf("%d", &x); a[x] = 1; } int nn = 1, ln = 0; int nw = (n * 5) + 1; while(nn < nw) { nn *= 2; ln++; } fft(a, nn, ln, false); forn(i, nn) a[i] = binPow(a[i], n / 2); fft(a, nn, ln, true); int ans = 0; forn(i, nn) ans = norm(ans + binPow(a[i], 2)); printf("%d\n", ans); return 0; }

转载于:https://www.cnblogs.com/storz/p/10229440.html