题目
请计算C[k]=sigma(a[i]*b[i-k]) 其中 k < = i < n ，并且有 n < = 10 ^ 5。 a,b中的元素均为小于等于100的非负整数。
输入格式
第一行一个整数N,接下来N行，第i+2..i+N-1行，每行两个数，依次表示a[i],b[i] (0 < = i < N)。
输出格式
输出N行，每行一个整数，第i行输出C[i-1]。
输入样例
5
3 1
2 4
1 1
2 4
1 4
输出样例
24
12
10
6
1
题解
和2179几乎一模一样
由于卷积的定义要求下标之和为常数，我们尝试将原式变形，发现只要将a或者b反过来存就可以了
#include<iostream>
#include<cmath>
#include<cstdio>
#include<cstring>
#include<complex>
#include<algorithm>
#define pi acos(-1)
#define LL long long int
#define REP(i,n) for (int i = 1; i <= (n); i++)
#define Redge(u) for (int k = h[u],to; k; k = ed[k].nxt)
#define BUG(s,n) for (int i = 1; i <= (n); i++) cout<<s[i]<<‘ ‘; puts("");
using namespace std;
const int maxn = 400005,maxm = 100005,INF = 1000000000;
inline int read(){
int out = 0,flag = 1; char c = getchar();
while (c < 48 || c > 57) {if (c == ‘-‘) flag = -1; c = getchar();}
while (c >= 48 && c <= 57) {out = (out << 3) + (out << 1) + c - ‘0‘; c = getchar();}
return out * flag;
}
typedef complex<double> E;
E a[maxn],b[maxn];
int n,m,L,R[maxn];
void fft(E* a,int f){
for (int i = 0; i < n; i++) if (i < R[i]) swap(a[i],a[R[i]]);
for (int i = 1; i < n; i <<= 1){
E wn(cos(pi / i),f * sin(pi / i));
for (int j = 0; j < n; j += (i << 1)){
E w(1,0);
for (int k = 0; k < i; k++,w *= wn){
E x = a[j + k],y = w * a[j + k + i];
a[j + k] = x + y; a[j + k + i] = x - y;
}
}
}
if (f == -1) for (int i = 0; i < n; i++) a[i] /= n;
}
int main(){
n = read(); n--;
for (int i = 0; i <= n; i++){a[n - i] = read();b[i] = read();}
m = n << 1; for (n = 1; n <= m; n <<= 1) L++;
for (int i = 0; i < n; i++) R[i] = (R[i >> 1] >> 1) | ((i & 1) << (L - 1));
fft(a,1); fft(b,1);
for (int i = 0; i <= n; i++) a[i] *= b[i];
fft(a,-1);
for (int i = (m >> 1); i >= 0; i--) printf("%d\n",(int)(a[i].real() + 0.1));
return 0;
}