卷积全家桶FFT快速傅里叶变换应用场景:有\(2^n-1\)次多项式\(f(x),g(x)\),求多项式\(f(x)g(x)\)的各项系数.换言之,有长为\(2^n\)数组\(f,g\),(下标从\(0\)开始),求长为\(2^{n+1}-1\)数组\(h\)满足\(h_i=\sum_{j=0}^{i}f_jg_{i-j}\)(超出\(f,g\)下标范围的地方用\(0\)补全).